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of discount. 



DYNAMO ELECTRIC 
MACHINERY; 

ITS CONSTRUCTION, DESIGN, 
AND OPERATION 

DIRECT-CURRENT MACHINES 



BY 

SAMUEL SHELDON, A.M., Ph.D., D.Sc. 

PROFESSOR OF PHYSICS AND ELECTRICAL ENGINEERING AT THE POLYTECHNIC 

INSTITUTE OF BROOKLYN AND PAST-PRESIDENT OF THE AMERICAN 

INSTITUTE OF ELECTRICAL ENGINEERS 

AND 

ERICH HAUSMANN, E.E., M.S. 

INSTRUCTOR IN PHYSICS AND ELECTRICAL ENGINEERING AT THE 

POLYTECHNIC INSTITUTE OF BROOKLYN, AND ASSOCIATE 

OF THE AMERICAN INSTITUTE OF 

ELECTRICAL ENGINEERS 

EIGHTH EDITION, COMPLETELY REWRITTEN 




NEW YORK: 

D. VAN NOSTRAND COMPANY 

LONDON 

CROSBY LOCKWOOD & SON 

1910 



& 



^cfi 



s* 



\0 



Copyright, 1900, by 
D. VAN NOSTRAND COMPANY 



Copyright, 1910, by 
D. VAN NOSTRAND COMPANY 



/K 



Stanbope ipress 

F. H. GILSON COMPANY 
BOSTON. U.S.A. 

7 

/ 
©CLA2716S8 



PREFACE. 

The object aimed at in the preparation of the first 
edition of this book has been kept in view and has con- 
trolled the preparation of this eighth edition. This has 
been the production of a text -book for the use of students 
pursuing electrical or non-electrical engineering courses. 
The method of presentation is considered as especially 
adapted for classroom exercises, which consist of recita- 
tions, computations, and occasional lectures, and which are 
supplemented by laboratory exercises, the two being cor- 
related with a view to training the mind of the student 
and adding somewhat to his knowledge. It will be found 
that in treatment the sequence is such that parts which it 
may seem undesirable to require from other than electrical 
engineering students may be omitted without introducing 
a discontinuity in the matter which remains. 

With the exception of the first two chapters, the book 
has been entirely rewritten; nearly two hundred of its illus- 
trations are new, most of them having been specially drawn 
to make clear methods of construction or characteristics of 
operation; and it has been considerably extended in scope. 
In the new matter will be found a set of problems at the 
end of each chapter, a presentation of the theory of com- 
mutation, means for the predetermination of the operating 
characteristics of direct-current generators and motors, a 
discussion on storage batteries from the engineering point 
of view, a treatment of the theory of balancers and of 



IV PREFACE. 

boosters, and a discussion of costs, prices, and operating 
expenses of machines and plants. 

The chapters on the design of machines and on tests, 
which appeared in the former editions, have been omitted, as 
these subjects require for their adequate treatment more 
space than one would be warranted in giving them in a 
book of this character. 

Polytechnic Institute, 
Brooklyn, New York, 
June i, 19 10. 



CONTENTS. 



CHAPTER I. 



Electrical Laws and Facts. 

Art. Page 

i. Mechanical Units i 

2. Electrical Units 3 

3. Ohm's Law , 4 

4. Resistance of Conductors 5 

5. Divided Circuits 8 

6. Power of Electric Current 10 

7. Heat Developed by a Current 11 

8. Insulating Materials 11 

9. Test of Dielectric Strength 13 

Problems 16 

CHAPTER II. 

Magnetic Laws and Facts. 

10. Strength of Magnet Pole 18 

11. Magnetic Field and Lines of Force 18 

12. Intensity of Magnetic Field 19 

13. Magnetic Potential 20 

14. Permeability 20 

15. Electro-magnetic Induction 21 

16. Direction of Induced E.M.F 23 

17. Inductance 24 

18. Growth of Current in an Inductive Circuit 26 

19. Decay of Current in an Inductive Circuit 27 

20. Quantity of Electricity Traversing a Circuit Due to a Change of 

Flux Linked with it 29 

21. Work Performed by a Conductor Carrying a Current and Moving 

in a Magnetic Field 29 

22. Force Exerted between a Field and a Conductor Carrying a 

Current 30 

v 



Vl CONTENTS. 

Art. Page 

23. Magnetomotive Force of a Circular Circuit Carrying a Current 31 

24. The Toroid 32 

25. Magnetization Curves 33 

26. Reluctance and Permeance 36 

27. Relation between Magnetomotive Force, Magnetic Flux, and 

Reluctance 37 

28. Hysteresis 38 

29. Eddy Currents 42 

Problems 43 



CHAPTER III. 

Armatures. 

30. Dynamos 45 

31. Principle of Action of a Generator 45 

32. The Function of the Commutator 46 

33. Electromotive Force Generated 48 

34. The Armature 52 

35. The Field Magnets 53 

36. Armature Windings 55 

37. Multiplex Armature Windings 62 

38. Equalizing Connections 65 

39. E.M.F. Equation of Dynamos 66 

40. Core Construction 67 

41. Armature Coils 73 

42. Commutators 76 

43. Brushes and Brush Holders 81 

44. Shafts and Bearings 83 

Problems 86 



CHAPTER IV. 

Field Magnets. 

45. Field-Magnet Frames 88 

46. Methods of Field Excitation 92 

47. Magnetic Leakage 94 

48. Calculation of Exciting Ampere-Turns 96 

49. Field Coils 104 

Problems 108 



CONTENTS. vil 



CHAPTER V. 

Armature Reaction. Commutation. 

Art. Page 

50. Armature Reaction no 

51. Cross-Magnetizing Effect of Armature Current in 

52. Demagnetizing Effect of Armature Current 113 

53. Compensation for Armature Reaction 115 

54. Devices for Reducing Armature Reaction 118 

55. Commutation 121 

56. Time of Commutation 125 

57. Calculation of Reactance Voltage 127 

58. Conditions for Good Commutation 134 

Problems 138 



CHAPTER VI. 

Generators. 
Efficiency of Operation. 

59. Capacity of a Dynamo 140 

60. Heating of Dynamos 142 

61. Output Coefficients 145 

62. Losses in Armature Cores 146 

63. Armature Copper Loss 148 

64. Pole-Face Losses 149 

65. Excitation Loss 151 

66. Bearing Friction and Windage 151 

67. Commutator Loss 152 

68. Temperature Elevation 153 

69. Efficiency 155 

70. Coefficient of Conversion 158 

71. Economic Coefficient 158 

72. Magnetos 159 

73. Constant-Potential and Constant-Current Supply 160 

Constant-Potential Generators. 

74. Characteristic Curves of Shunt- Wound Generators 162 

75. Voltage Regulation 164 

76. Hand Regulation 165 

77. Field Rheostats 166 



Vlll CONTENTS. 

Art. Page 

78. Self-Regulation 171 

79. Characteristic Curves of Compound- Wound Generators 172 

80. Railway and Lighting Generators 173 

81. Three- Wire Generators 180 

82. Homopolar Dynamos 184 

Constant-Current Generators. 

83. Characteristic Curves of Series- Wound Generators 187 

84. Power Lines 189 

85. Series- Wound Generators 189 

86. The Brush Machine 193 

87. The Excelsior Arc-Light Generator 198 

88. The Thomson-Houston Dynamo 200 

89. Western Electric Arc-Light Dynamo 204 

Problems 207 



CHAPTER VII. 

Motors. 

90. Principle of Action of a Motor 210 

91. Direction of Rotation 211 

92. Torque Exerted by a Motor 213 

93. Counter Electromotive Force 214 

94. Armature Reactions . 216 

95. Power of Motors 216 

Shunt Motors. 

96. Speed of Shunt Motors 217 

97. Starting of Shunt Motors 225 

98. Design of Starting Rheostats 230 

99. Speed Regulation 232 

100. Characteristic Curves of Shunt Motors 233 

101. Industrial Applications of Shunt Motors s 236 

Series Motors. 

102. Series Motors 239 

103. Characteristic Curves of Series Motors 241 

104. Railway Motors 241 

105. Railway Motor Control 251 

106. Motors for Automobiles 258 



CONTENTS. IX 

Art. Page 

107. Motors for Rolling Mills 260 

108. Crane Motors 261 

109. Compound- Wound Motors 263 

Problems 265 



CHAPTER VIII. 

Dynamotors, Motor-Generators, Boosters, and Storage Batteries. 

1 10. Dynamotors 266 

in. Motor- Generators 273 

112. Boosters 279 

113. Storage Batteries 287 

Problems 292 

CHAPTER IX. 

Central-Station Equipment. 

114. Paralleling of Generators 294 

115. Parallel Operation of Motors 299 

116. Switches 300 

117. Fuses 302 

118. Circuit Breakers 303 

119. Measuring Instruments 305 

1 20. Switchboards 312 

121. Works Cost 314 

122. Selling Prices 315 

123. Plant Costs 318 

124. Operating Expenses 319 

125. Cost of Electrical Energy 320 

Problems 322 



DYNAMO ELECTRIC MACHINERY. 



CHAPTER I. 

ELECTRICAL LAWS AND FACTS. 

i. Mechanical Units. — Force is that which tends to 
produce, alter, or destroy motion. The units of force are 
the dyne and the poundal. The dyne is that force which, 
acting on a one-gram mass for one second, will tend to 
produce a velocity of one centimeter per second. The 
poundal is that force which, acting on a mass of one pound 
for one second, will tend to produce a velocity of one foot 
per second. The weight of a pound mass is frequently 
taken as a unit of force, and is called, for brevity, a pound. 
A force of one pound is approximately equal to 32.2 
poundals. 

Work is the production of motion against resistance. 
The units of work are the foot-pound and the erg. The 
foot-pound is the work done in lifting a body weighing one 
pound one foot vertically. The erg is the work performed 
by a force of one dyne in moving a body one centimeter 
in the direction of the force. The joule is a larger unit 
much used, and is equal to io 7 ergs. 

Energy is the capacity to do work. It is expressed in 
the same units as work. The two classes of energy are 
Kinetic energy and Potential energy. A body possesses 



2 DYNAMO ELECTRIC MACHINERY. 

kinetic energy in virtue of its motion, while potential energy 
is due to the separation or the disarrangement of attracting 
particles or masses. A wound-up spring has potential 
energy because of the strained positions of the molecules, 
while a weight raised to a height has potential energy 
because of the separation of its mass from the attracting 
mass of the earth. The potential energy of a body is 
measured by the work required to put the body into its 
strained condition. The kinetic energy of a body is pro- 
portional to its mass and to the square of its velocity, or 

Kinetic Energy = , 

2 g 

since the mass of a body is equal to its weight divided by 
the acceleration due to gravity. Kinetic energy will be 
expressed in ergs when J^is the weight in grams of the 
body whose velocity is v centimeters per second, and when 
g is 98 1 cm. per second per second. If W be the weight 
in pounds, v the velocity in feet per second, and g is 32.2 
feet per second per second, then the kinetic energy is ex- 
pressed in foot-pounds. 

Power is the rate of performance of work. Its units are 
the horse-power and the watt. A horse-power is 33,000 
foot-pounds per minute. A watt is one joule per second. 
One horse-power is equivalent to 746 watts. Representing 
the torque or twisting moment of a machine in pound-feet 
by T, and its angular velocity in radians per second by co 
= 2 7i V/60, where V is the number of revolutions per 
minute, then the horse-power of the machine is 

up 6O WT _ 2 7T VT 

33000 33000' 
In a belt-driven machine the torque in the shaft is equal to 



ELECTRICAL LAWS AND FACTS. 3 

the difference in tension of the two sides of the belt multi- 
plied by the radius of the pulley, that is, T = (F — F') r 
pound-feet. 

2. Electrical Units. — Since distinction must continually 
be made between absolute or c.g.s. units and practical units, 
throughout this work capital letters will be used for quan- 
tities expressed in practical units, and lower-case letters for 
quantities expressed in absolute units. 

The absolute unit of current is such that, when flowing 
through a conductor of one centimeter length, which is 
bent into an arc of one centimeter radius, it will exert a 
force of one dyne on a unit magnet pole (§ 10) placed at 
the center. The practical unit of current, the ampere, is 
one-tenth the magnitude of the absolute unit. 

The absolute unit of quantity is that quantity of elec- 
tricity which in one second passes any cross-section of a 
conductor in which the absolute unit of current is flowing. 
The practical unit of quantity is one-tenth of the absolute 
unit, and is called the coulomb. For large quantities the 
ampere-hour and the mega-coulomb, the latter being equal 
to a million coulombs, are units frequently used ; and for 
small quantities the micro-coulomb, equal to one-millionth 
of a coulomb, is often used. 

The absolute unit of difference of potential exists between 
two points when it requires the expenditure of one erg of 
work to move an absolute unit quantity of electricity from 
one point to the other. The practical unit of difference 
of potential, the volt, is io 8 times as large as the absolute 
unit. 

It is convenient and rational to make a distinction be- 
tween electromotive force and difference of potential. 
Electromotive force is produced when a conductor cuts 



4 DYNAMO ELECTRIC MACHINERY. 

magnetic lines of force, or when the electrodes of a pri- 
mary battery are immersed in a solution. But a difference 
of potential may exist due to the flow of an electric cur- 
rent. Between any two points of a conductor carrying a 
current there is that which would send a current through 
an auxiliary wire connecting these points, and it is called 
difference of potential. If the current in the original con- 
ductor be doubled, the difference of potential between the 
same two points will be doubled, showing that this differ- 
ence of potential exists because of the current flowing in 
the original conductor. The word pressure is used either 
for difference of potential or for E.M.F. with obvious 
relevancy. 

The absolute unit of resistance is offered by a body when 
it allows an absolute unit of current to flow along it be- 
tween its two terminals, when these are maintained at unit 
(absolute) difference of potential. The practical unit of 
resistance, the ohm, is io 9 times as large as the absolute 
unit. The megohm, equal to a million ohms, and the 
microhm, equal to one-millionth of an ohm, are units fre- 
quently used. 

3. Ohm's Law. — The relation between the current, 
electromotive force, and resistance of a simple circuit is 
given by Ohm's law, in absolute units, as 

e 
1 = -. 
r 

Since the current in amperes is ioz, the E.M.F. in volts 
is io -8 *, and the resistance in ohms is 10-V, Ohm's law may 
be expressed in practical units by the formula 

'-f 



ELECTRICAL LAWS AND FACTS. 5 

where / is the number of amperes flowing in an undivided 
circuit, E the algebraic sum of all the electromotive forces 
in that circuit in volts, and R the sum of all the resistances 
in series in that circuit expressed in ohms. 

The form of the equation E — IR, as applied to a por- 
tion of a circuit, is much used under the name of Ohm's 
law. In this case, however, E is not E.M.F., but differ- 
ence of potential, as explained in the last article. 

If, in a house lighted by electricity, the service maintains 
a constant pressure of ioo volts at the mains where they 
enter from the street, and no lights be turned on, then at 
every lamp socket in the house there will be a pressure of 
ioo volts. If now a lamp be turned on, it will be working 
on less than ioo volts, because of the drop or fall of po- 
tential. If many lamps be turned on, a considerable drop 
may occur. The drop is caused by the resistance of the 
wires carrying the current from the place of constant po- 
tential to the place where it is used, and the volts lost have 
been consumed in doing useless work, i.e. heating the wires. 
That the drop is proportional to the current flowing is 
shown by a simple application of Ohm's law. 

Let R be the resistance of the line, and E d the volts 
drop caused thereby when a current / flows. Then 

E t =IR, 

from which it is evident that the drop varies as the cur- 
rent when the resistance of the line is constant. 

4. Resistance of Conductors. — The resistance R of a con- 
ductor is expressed by the formula 

'-$■ 

where p is a constant called the resistivity, and depending 



6 DYNAMO ELECTRIC MACHINERY. 

upon the material and the temperature of the conductor, /is 
the length, and A the cross-section of the conductor. The 

reciprocal of the resistivity, — is called the conductivity of a 

P 
substance. 

If, in the foregoing expression for R, the centimeter and 
square centimeter be the units of length and cross-section 
respectively, and the resistance is desired in ohms, then 
p must be the resistance between opposite faces of a cen- 
timeter cube of the given material, and this is called its 
specific resistance. Areas of conductors are frequently ex- 
pressed in terms of a unit, ciratlar mil, equal to the area 
of a circle y^oo i ncn m diameter. If area be so expressed 
and if / be the length in feet of the conductor, then p must 
be the resistance of a portion thereof one foot long and 
one circular mil in cross-section, i.e. of one mil-foot, so 
that R may be in ohms. The resistivities of various 
metals at o° Centigrade are given in the following table : 



RESISTIVITIES. 



MATERIAL 



Copper (soft) . . . 
Aluminum (soft) . . 
Iron (soft) . . . . 
Platinum . . . . 

Steel 

German Silver (18%) 
Manganin . . . . 
German Silver (30%) 
"Advance". . . . 

"la la" 

"Climax" . . . . 
" Superior " . . . . 



SPECIFIC RESIST- 
ANCE IN MI- 
CROHMS AT 0°C. 



1-594 

2-55 
8.7 
8.98 
13.0 

34 
43 
45 
49 
50 
85 
86 



RESISTANCE PER 
MIL-FOOT, IN 
OHMS AT O C 



9.6 
154 
524 
541 

78 
204 

259 

271 

295 
30I 

512 

518 



As rectangular conductors are much used in armatures 
and upon switchboards, it frequently becomes necessary to 



ELECTRICAL LAWS AND FACTS. 7 

express their cross-sections in circular mils. Since the 
cross-secticn of a circle having a diameter of T oV o inch is 
0.00000078$ square inches, the equivalent cross-section of 
a conductor expressed in circular mils is equal to its cross- 
section in square inches divided by 0.000000785, and 
the cross-section in circular mils is equal to 1273236 times 
its cross-section in square inches. 

The resistivity of a conductor depends upon its physical 
condition and upon its purity. Thus, for example, the 
resistivity of soft copper is two per cent lower than that of 
hard copper. The resistivity of an alloy is usually greater 
than that of any of its constituents, consequently the ad- 
mixture of a small percentage of one metal with another 
usually implies a higher resistivity. 

The American Institute of Electrical Engineers has 
adopted as its standard resistivity for soft copper one given 
by Matthiessen. A wire of standard soft copper, of uni- 
form cross-section, of one meter length, and weighing one 
gram, should have a resistance of o. 141 729 international 
ohms at o° C. A commercial copper showing this resis- 
tivity is said to have 100 per cent conductivity. Copper is 
frequently obtained having a conductivity of 102 per cent. 
It is in these cases almost invariably electrolytic copper. 

The resistance of conductors depends upon temperature. 
Rise of temperature causes an increase of resistance in all 
pure metals, and the rate of increase is approximately the 
same for each. Representing the increase of resistance 
per unit resistance at o° C. and unit rise in temperature by 
a, called the temperature coefficient, and the resistance of a 
conductor at o° C. by R , then its resistance at any temper- 
ature T may be expressed by 

R T -R t (i +aT). 



8 DYNAMO ELECTRIC MACHINERY. 

While it is sufficient for engineering purposes to consider 
the temperature coefficient of any conductor constant, it 
should be remembered that this coefficient varies slightly 
at different temperatures. 

The average value of the temperature coefficient of cop- 
per is 0.0042, that is, between any initial and final temper- 
ature copper increases its resistance by 0.42 per cent of 
its resistance at zero degrees Centigrade for each degree 
rise of temperature. 

Many alloys have a very small temperature coefficient, 
and are thus desirable for resistances in measuring instru- 
ments. Acid and salt solutions, carbon, hard rubber, and 
glass have negative temperature coefficients. 

5. Divided Circuits. — When portions of an electric cir- 
cuit are connected in series, the total resistance of the cir- 
cuit is equal to the sum of the resistances of the separate 
portions. To determine the equivalent resistance of a 
number of resistances connected in parallel, let / be the 
current flowing in the undivided part of the circuit shown 



/?/ 

\ 



I 




Fig. 1. 



in Fig. I, and let I x and I 2 be the currents flowing in the 
resistances R x and R 2 respectively. Then 



ELECTRICAL LAWS AND FACTS. 9 

and, since the potential difference, E, across each branch 
circuit is the same, by Ohm's law 

/ 1 =f i and/ 2 = f; 

whence I x : I 2 : :~ : ^- • 

The currents in the branches of a divided circuit are in- 
versely as the resistances of the branches. 

If R e be a single resistance, that, when substituted for 
the shunted resistances R x and R 2 will leave /unchanged, 

then / = — ; consequently 

Re 



or R e = 



The resistance equivalent to a number of shunted resistances 
is equal to the reciprocal of the sum of the reciprocals of the 
separate resistances. 

The distribution of current through the elements of a 
network of conductors, no matter how complex, may be 
determined by the aid of the following two laws due to 
Kirchhoff : — Law I. — The algebraic sum of the currents 
meeting at any point of a netzvork is zero. Law II. — In 
any mesh of a network the algebraic sum of the IR drops 
is equal to the algebraic sum of the electromotive forces. 
For example, if E be the electromotive force of the battery 
indicated in Fig. 1, according to the first law 

r- /,-/,-<>, (1) 



E 

Re 


E 

~ R, 


+ f 




R 


+ R> 


I 




*i 


z + 


I 

~R. 



IO DYNAMO ELECTRIC MACHINERY. 



and from the second law 






E = 


IR + I x R l9 


and 


E = 


- IR + h R , 


From (2) and (3) 


'X- 


*Wr 


Hence from (1) 


I- 


. / _ R A - 
1 z, ' 


or 


I,= 





(2) 

(3) 



o, 



6. Power of Electric Current. — If a difference of poten- 
tial of e absolute units exist between two points, the trans- 
fer of an absolute unit quantity of electricity from one 
point to the other requires the expenditure of e ergs of 
work. Since the volt is equal to io 8 absolute units of 
potential difference, and the coulomb is equal to io -1 abso- 
lute units of quantity, it follows that the work performed in 
transferring one coulomb of electricity under a difference of 
potential of one volt is io 7 ergs or one joule. A current 
of I amperes flowing for / seconds represents It coulombs 
of electricity, and if these be transferred under a potential 
difference of E volts, the work done in joules will be 

w = En. 

The rate of working, or the power, is 

W 
P = — = EI, 
t 

where P is expressed in watts, i.e. joules per second. 
Since, from Ohm's law, E= IR, by substitution 

P = PR. 

For commercial currents and voltages the watt is a need- 
lessly small unit, hence the kilowatt (= 1000 watts) is 



ELECTRICAL LAWS AND FACTS. II 

generally used to express electrical power. It is repre- 
sented by the abbreviation k.w. The horse-power is equal 
to 746 watts, or approximately three-fourths of a k.w. 

7. Heat Developed by a Current. — When a current / 
is maintained in a circuit of resistance R, the work per- 
formed is converted into heat. The work thus done per 
second, or the power expended, will be I 2 R watts. Since 
this production of heat is often of no service, this expendi- 
ture of power is generally called the I 2 R loss. 

This production of heat causes a rise of temperature in 
the conductor, and the temperature will continue to rise till 
the heat generated per second by the I 2 R loss is exactly 
counterbalanced by the rate of dissipation of heat by con- 
duction, convection, and radiation. 

The inherent resistances of electrical machines involve 
the production of heat in their operation (as do also fric- 
tion and reversal of magnetism), which causes a rise of tem- 
perature. As insulating materials can survive only moder- 
ately high temperatures, such machines must be designed 
to operate without becoming too hot. This is accomplished 
by decreasing the PR loss, by increasing the radiating sur- 
face, and by improving ventilation. 

8. Insulating Materials. The desirable properties of 
materials which are to be used for insulating various elec- 
trical conductors from each other are : (a) a high insulation 
resistance and this resistance should remain high over a 
considerable range of temperature ; (b) a dielectric strength 
sufficient to preclude any possibility of perforation by volt- 
ages liable to exist between the conductors which they 
separate, and this strength must also persist throughout all 
probable ranges of temperature ; (c) such physical proper- 
ties as will permit of mechanical manipulation ; (d) non- 



12 DYNAMO ELECTRIC MACHINERY. 

alteration of chemical constitution when subjected to high 
temperatures and operating conditions. 

As no one insulating material possesses all of these desir- 
able properties, for any particular purpose that insulating 
material should be chosen which is best suited for the given 
conditions. In this choice, available space and cost of the 
insulation are also determining factors. 

The dielectric strength of an insulating material is 
measured by the voltage which must be applied to it in 
order to cause its rupture. The dielectric strength depends 
upon the thickness of the dielectric, the form of the opposed 
conducting surfaces, and the manner in which the E.M.F. 
is applied, whether gradually, suddenly, or periodically 
varying. It has been stated that the dielectric strength 
approximately varies inversely as the cube root of the 
thickness, showing that a thin sheet is relatively stronger 
than a thick one of the same material. For example, the 
dielectric strength of mica when I mm. thick is 610 kilovolts 
per centimeter, but when o. I mm. thick it is 1 150 kilovolts 
per centimeter. 

Mica possesses the highest insulation resistance and the 
greatest dielectric strength of insulating materials. It does 
not absorb moisture and its chemical constitution is un- 
affected by high temperatures. It is not, however, me- 
chanically strong. 

Sheets of insulation made up from pieces of scrap mica 
cemented together by linseed oil or preparations of shellac, 
when carefully constructed with lapped joints, exhibit 
nearly as good insulating and dielectric properties as sheet 
mica. While not perfect mechanically, these sheets permit 
of bending better than pure mica. 

For insulating purposes where considerable flexibility 



ELECTRICAL LAWS AND FACTS. 1 3 

is essential, micanite paper and micanite cloth are well 
adapted. These materials consist of small pieces of mica 
in combination with paper or various kinds of fabric. 

Preparations of fibrous materials with linseed oil, which, 
after being dried, have been thoroughly baked, are fairly 
good insulators. As water is generally present in their 
pores, their insulation resistance, upon heating, decreases 
until the temperature has reached ioo° C, and then it in- 
creases. These preparations are mechanically flexible. 
Preparations of fibrous material with shellac are good in- 
sulators, but crack upon bending. 

Vulcanized fibers are made by treating paper fiber chemi- 
cally, and, when dried, they have a fairly high insulation 
resistance, but they readily absorb moisture, and, upon dry- 
ing, are liable to warp and twist. They furthermore be- 
come brittle when heated. 

Hard rubber is a good insulator and possesses the desir- 
able mechanical qualifications, but it does not withstand 
moderately high temperatures, for at Jo° C. it becomes 
soft and melts at 8o° C. Its employment is limited, 
therefore, to apparatus to be used at comparatively low 
temperatures. 

Asbestos, a fairly good insulator, is used principally be- 
cause of its incombustibility. Vulcabeston, which is a 
preparation of asbestos and rubber, exhibits good insulating 
and mechanical qualities, and is especially fitted for higher 
temperatures. Asbestos and vulcabeston are much used 
in electric heating apparatus. 

9- Test of Dielectric Strength. — In order to test the 
voltage necessary to break down a sample sheet of insulat- 
ing material, the sample is placed between two flat metal- 
lic surfaces which are connected respectively with the two 



14 



DYNAMO ELECTRIC MACHINERY. 



terminals of a high-voltage transformer, whose voltage can 
be varied at will. The sample should project considerably 
beyond the edges of the metallic surfaces, so that no dis- 
charge can take place from one terminal to the other around 
the sample under test. The test voltage is applied and is 
gradually increased until the material punctures. 

Practical average values of dielectric strengths over the 
thicknesses stated of various insulating materials are given 
in the following table : 



MATERIAL 


THICKNESS 
IN MM. 


DIELECTRIC 
STRENGTH IN 

EFFECTIVE 

SINUSOIDAL 

KTLOVOLTS 

PER CM. 


Air 


°-5 

I.O 

5-o 

IO.O 

0.5-1.5 
0.1-0.3 

2.0-6.0 

i-5-75-o 

0.05-3.0 

0.2-0.5 

0.2-0.6 

0.2-0.6 

0.05-0.2 

0.02-0.2 

1.0-2.5 

0.8-2.0 


5°-3 

43- 6 

33-5 

29.8 

25-10 

no 

250-170 

510-400 

1300-500 

400 

150 

80 

360 

200-150 

80-20 

80 




« 


« 


Asbestos 


Cotton 


Glass 


Hard rubber 


Mica 

Micanite ...... 


" paper 

" cloth 

Paper (paraffined) 

Silk 

Vulcabeston .... 


Vulcanized fiber , . 





For measuring the test voltage a spark gap having sharp 
needle-point terminals is connected in parallel with the 
sample under test. The distance between the needle-points 
is adjustable so as to limit the voltage which can be im- 
pressed upon the conductors on each side of the insulating 
material. In carrying out the test, the needle-points are 
adjusted at a certain minimum distance apart. The voltage 



ELECTRICAL LAWS AND FACTS. 



15 



impressed upon the terminals is raised until a spark passes 
between the points. The air gap is then increased in 
length, and the operation repeated until the sample breaks 
down, and the spark passes through it instead of across 
the air gap. The length of the air gap is measured and the 
break-down voltage may then be obtained from the curve 
of Fig. 2, which shows the effective sinusoidal voltages 



275 

250 

225 

200 

H 175 
_J 
O 
>150 

_j 

*125 
100 
75 
50 
25 

























































































































































































/ 






























































































































/- 





























12 14 16 U 
INCHES 

Fig. 2. 



20 22 24 26 



corresponding to the sparking distances in air between 
opposed sharp needle-points. 

The test voltage to be applied in determining the suita- 
bility of insulation in commercial apparatus depends upon 
the kind and the size of the apparatus, its normal voltage, 
and the service for which it is designed. The following 
voltages for testing insulation of apparatus and cables by a 



i6 



DYNAMO ELECTRIC MACHINERY. 



continuous application for one minute are recommended by 
the American Institute of Electrical Engineers : 



RATED TERMINAL VOLTAGE OF CIRCUIT 



Not exceeding 400 volts 
400 volts-800 volts . 
800 volts- 1 200 volts 

1200 VOltS-2500 VoltS . 

2500 volts and over . . 



RATED OUTPUT 


Under 10 K.W. 
ioK.W.and over 
Under 10 K.W. 
10 K.W. and over 

Any 
Any 
Any 



TESTING VOLTAGE 



1000 volts 
1500 " 
1500 " 
2000 " 

3500 " 

5000 " 
Double normal 
rated voltage 



The test voltage should be applied successively between 
each electric circuit and surrounding conductors and also 
between adjacent electric circuits. High-voltage tests 
should be made at the temperatures assumed during normal 
operation. 

PROBLEMS. 

1. How much work is done by a pump in raising 2500 gal- 
lons of water from a mine 200 feet deep? If this is accom- 
plished in 25 minutes, what is the power of the pump expressed 
in horse-power ? 

2. Calculate the kinetic energy of an electric locomotive 
weighing 95 tons when running at a velocity of 50 miles per 
hour. 

3. The hot resistance of a 100-candle-power carbon incan- 
descent lamp is 45 ohms. How much current does the lamp 
take when connected to 120-volt mains ? 

4. What must be the E.M.F. of a generator to supply a group 
of 50 lamps connected in parallel, each requiring h ampere, 
the resistance of the generator being 0.1 ohm, so that each lamp 
of 220 ohms resistance shall receive its full current, assuming 
the line wires to have a resistance of 0.5 ohm ? 



PROBLEMS. 



17 



5. Determine the resistance of one mile of copper wire 0.325 
inch in diameter at zero degrees Centigrade. 

6. Calculate the resistance at i5°C. of a mile of track rail 
weighing 70 pounds per yard, taking 120 ohms as its resistance 
per mil-foot at that temperature. Specific gravity of track rail 

=7.8. 

7. Find the resistance at 700 C. of a platinum wire two 
meters long and one millimeter in diameter ; the temperature 
coefficient being 0.0036. 

8. When four conductors of 4, 8, 10, and 16 ohms resistance 
respectively are joined in parallel to the terminals of a battery 
whose E.M.F. is 20 volts on open circuit and whose internal 
resistance is 3 ohms, how much current will flow in each 
conductor ? 

9. Resistances of 2, 3, 4, 6, 8, and 10 ohms respectively are 
connected as shown in Fig. 3, the number adjacent to each 



AAAA/V 



A/WW 



WW — M/ww- 



AAA/W 



■AAAA/V — ' 



Fig. 3. 



branch representing its resistance. What will be the potential 
difference across each resistance and the current therein when 
the terminals A, B are connected to 50-volt mains ? 

10. What power is expended in the 100-candle-power lamp 
of problem 3 ? Express the result in watts and in horse-power. 
When electrical energy costs 10 cents per kilowatt-hour, how 
much does it cost to operate the lamp for three hours ? 



l8 DYNAMO ELECTRIC MACHINERY. 



CHAPTER II. 

MAGNETIC LAWS AND FACTS. 

io. Strength of Magnet Pole. — A unit magnet pole is 
one which will repel an equal like pole, when at a distance 
of one centimeter in vacuum or in air,* with a force of one 
dyne. 

It follows from this definition that a pole m units strong 
will repel a like unit pole with a force of m dynes. The 
force exerted between two magnetic poles varies inversely 
as the square of the distance between them. Hence the 
force in dynes exerted between two magnetic poles of 
strengths m and m' when d centimeters apart in air is 
denned by the equation 

■p _ mm! 
~ ~d r ' 

ii. Magnetic Field and Lines of Force. — The space 
around a magnet where its action is felt is termed the field 
of that magnet, This field may conveniently be considered 
as permeated by lines of force. These lines represent by 
their direction the direction of the force exerted by the 
magnet, and by their closeness to each other show the 
magnitude of this force. 

The directions of the lines of force in the vicinity of a 
magnet may be demonstrated by scattering iron filings 
over a glass plate laid on a magnet. Magnetic poles are 

* In this chapter air is considered to have the same magnetic properties 
as a vacuum. 



MAGNETIC LAWS AND FACTS. 19 

induced in the iron particles and the latter arrange them- 
selves parallel to the lines of. force, this arrangement being 
facilitated by gently tapping the glass plate. 

12. Intensity of Magnetic Field. — A magnetic field in 
air is said to have unit strength or intensity at any point 
therein when a unit magnet pole placed at that point is 
acted upon by a force of one dyne, or when a magnet pole m 
units strong is acted upon by a force of m dynes. There- 
fore the strength of a magnetic field in air, represented by 
JC, may be expressed as 

m 

By convention one line of force per square centimeter is 
considered to represent a field of unit strength, the square 
centimeter being on a surface that is at all points perpendic- 
ular to the lines cutting it. Hence the strength or inten- 
sity of any field in air, 3C, can be expressed by the number 
of lines of force per square centimeter. 

Suppose a sphere of one centimeter radius to be circum- 
scribed about a unit magnet pole. Another unit pole at 
any point on the surface of this sphere will be acted upon 
by a force of one dyne. Hence there exists unit field in- 
tensity at any point on this surface. But there are 4 n 
square centimeters on this surface, and each square centi- 
meter will be cut by one line of force. Therefore, there 
emanate from a unit magnet pole 4 n lines of force. Similarly 
a magnet pole of strength m sends out 4 nm lines of force. 
The total number of lines of force, or the total magnetic 
flux, represented by the symbol $, may therefore be ex- 
pressed as <f> = 4 nm. 

When a magnetic field has different intensities at various 
points in it, as is usually the case, it is called a non-uniform 



20 DYNAMO ELECTRIC MACHINERY. 

field ; and when it has everywhere the same intensity and 
direction, it is said to be a uniform field. 

13. Magnetic Potential. — The magnetic potential at 
any point is measured by the work that would be required 
to bring a unit magnet pole up to that point from an in- 
finite distance. 

The difference of magnetic potential between any two 
points is measured by the work in ergs required to carry a 
unit magnet pole from one to the other. 

14. Permeability. — The maintenance of the same dif- 
ference of magnetic potential between two points will result 
in more lines of force in iron than in air. Iron is then said 
to be more permeable than air, or to have a greater per- 
meability. If, for a given gradient of magnetic potential, 
3C lines of force per square centimeter be set up with air 
as a medium, and later at the same point (B lines with some 
other substance as a medium, then the ratio of (B to 30, 
expresses the permeability of that substance. This ratio 
is usually represented by the symbol ji, so that 

(B 

The permeability p expresses, therefore, the relative mag- 
netic conductivity of a substance compared with air. 

The total flux in the second substance is sometimes 
considered to be made up of two parts, the first consisting 
of that which would be present with air as a medium and 
the second being that which is added by the second 
substance. The total number of lines of force per square 
centimeter, (B, produced in a substance of permeability p. is 
called the flux density, or the induction per sqicare centi- 
meter. For air, vacuum, and most substances p = I. For 



MAGNETIC LAWS AND FACTS. 21 

iron, nickel, and cobalt p. has a higher value, reaching, in 
the case of iron, as high as 3000. Such substances are 
said to be paramagnetic, or simply magnetic. Bismuth, 
antimony, phosphorus, and a few other materials have a 
permeability very slightly less than unity ; these being 
known as diamagnetic substances. A substance for which 
[i is zero would insulate magnetism; but no such substance 
is known. 

One line of force, that is, a flux equal to — ■ that from an 

isolated unit pole, is called a maxwell. A flux density of 
one line of force, or one maxwell, per square centimeter, is 
called a gauss. The total magnetic flux, <l>, in maxwells, 
which passes through an area of A square centimeters, 
in which the flux-density is (B gausses, is given by the 
equation <S> = ©A 

15. Electro-Magnetic Induction. — In 1831 Faraday dis- 
covered that when a conductor was moved in a magnetic 
field, an electromotive force was set up in the conductor. 
This phenomenon is the foundation of all modern electri- 
cal engineering. 

An absolute unit of E.M.F. is produced when a conduc- 
tor cuts one line of force per second. If the conductor 
cuts two lines in the second, or one line in half a second, 
then two such units of electromotive force are produced. 
An E.M.F. of one volt is produced by the cutting of 
io 8 lines of force per second. 

If, in the short interval of time, dt seconds, d<& lines be 
cut, then during that interval the value of the induced 
E.M.F. will be 

d$ 

e = absolute units, 

dt 



22 



DYNAMO ELECTRIC MACHINERY. 



or, 



E = 



i d$ 



volts, 




io 8 dt 

the negative sign being used because the induced E.M.F. 
tends to send a current in such a direction as to demag- 
netize the field. When of no con- 
sequence the negative sign will 
hereafter be omitted. 

If a conductor, Fig. 4, / centi- 
meters long moves in a direction 
perpendicular to itself with a uni- 
form velocity of v centimeters per 
second across a uniform magnetic 
field having a flux density of (B 
gausses, the plane of its path 
making an angle a with the direc- 
tion of the lines of force, then the 
number of lines cut per second is (Rlv sin a, and, since the 
rate of cutting is uniform, the E.M.F. at any instant is 
e' = (St/v sin a. 
If there be a non-uniformity in the rate of cutting lines, 
due either to an uneven field or to an irregular motion, 
then the average value of the induced E.M.F. associated 
with the cutting of <J> lines in the time, t seconds, will be 

e« v = — absolute units, or E av = volts. 

av t av io 8 / 

If a circular loop of wire revolve about its diameter as 

an axis in a non-uniform magnetic field with a constant 

angular velocity, or if it revolve in a uniform field with a 

variable velocity, its sides cut lines of force at various rates. 

The instantaneous E.M.F. in the whole loop will be as 

before, f j$ 

e ~~di 9 



MAGNETIC LAWS AND FACTS. 



23 



where $ is the number of lines that links with, or that 
passes through, the loop. * If the loop be of n turns, then 
the pressure will be n times as great, or during the inter- 
val dt, 

ndfy 



E = 



io 8 dt 



volts. 



16. Direction of Induced E.M.F. — The direction of 
flow of a current induced in a closed circuit by moving it 
in a magnetic field is best represented by drawing the 
conventional representation of the three dimensions of 





Fig. 5. 



Fig. 6. 



space. If the flux be directed upwards, and the motion of 
the conductor be to the right, then the E.M.F. will tend 
to send a current toward the reader. If any one of these 
conditions be changed it necessitates the change of one of 
the others, and conversely the change of any two leaves 
the third unaltered. About the same idea is represented 
in Fleming's Rule, which is as follows : — 



24 DYNAMO ELECTRIC MACHINERY. 

Let the index finger of the right hand point in the di- 
rection of the flux, and the thumb in the direction of the 
motion. Bend the second finger at right angles with the 
thumb and index finger, and it will point in the direction 
of the E.M.F. 

Another rule is : — 

Stand facing a north magnetic pole. Pass a conductor 
downward. The current tends to flow to the left. 

17. Inductance. — An electric current produces a mag- 
netic field in the vicinity of the conductor carrying that 
current. The conductor may therefore be considered as 
encircled by lines of force. When the current is first 
started in such a conductor, these lines of force must be 
established. In establishing itself, each line is considered 
as having cut the conductor, or, what is equivalent thereto, 
been cut by the conductor. This cutting of lines of force 
results in the production of an electromotive force in the 
conductor, called the E.M.F. of self-induction. When the 
flow of current ceases, the surrounding lines of force col- 
lapse, cutting the conductor, thus also producing an electro- 
motive force of self-induction. The E.M.F. of self-induction 
is always a counter E.M.F., that is, its direction is such 
as to tend to prevent the change of current which 
causes it. 

The magnitude of this E.M.F. is dependent upon the 
rapidity with which the field is established or destroyed, 
and upon a constant called the self -inductance or the co- 
efficient of self-induction of the circuit. It is generally 
represented by the letter L, and is that coefficient by which 
the time rate of change of current in the circuit must be 
multiplied in order to give the E.M.F. induced in the cir- 
cuit. Its absolute value is numerically represented by the 



MAGNETIC LAWS AND FACTS. 2$ 

number of lines of force linked with the circuit per absolute 
unit of current in that circuit. Its practical unit is io 9 
times as large as the absolute unit, and is called the henry, 
A circuit having an inductance of one henry will have a 
pressure of one volt induced in it by a uniform change of 
current of one ampere per second. Hence the E.M.F. of 
self-induction may be written 

But, from §15, the E.M.F. induced in a loop of wire mov- 
ing in a magnetic field is 

~ nd$ . 

E = io -8 . 

dt 

Equating these expressions, there results 

d<$> 
L = n—= io -8 . 
dl 

Two circuits may exercise a mutually inductive action 
upon each other, and an E.M.F. may be induced in one by 
a change of current in the other. This is called an E.M.F. 
of mutual induction. In magnitude it depends upon the 
shape and position of the two circuits, and upon the char- 
acter of medium in which they are placed. It is also 
dependent upon a constant which is called the mutual 
inductance or coefficient of mutual induction of the two 
circuits. It is generally represented by the letter M. 
It is that coefficient by which the time rate of change of 
the current in one of the circuits is multiplied in order to 
give the E.M.F. induced in the other circuit. Its absolute 
value is numerically equal to the number of lines of force 
linked with one of the circuits per absolute unit of current 



26 DYNAMO ELECTRIC MACHINERY. 

in the other circuit. Its practical unit is the same as the 
practical unit of self-inductance, that is, the henry, and is 
io 9 times as large as the absolute unit. 

1 8. Growth of Current in an Inductive Circuit. — When 
a current is started in a circuit having resistance and in- 
ductance by impressing a constant E.M.F. upon its ter- 
minals, the self-induced pressure in that circuit tends to 
oppose the flow of the current and prevents it reaching its 
ultimate value immediately. At the instant of closing the 
circuit there is no current flowing, and let time be reckoned 
from this instant. At any subsequent instant, t seconds 
later, the impressed E.M.F. may be considered as the sum 
of two parts, E and E r . The first, E l} is that part which 
is opposed to, and just neutralizes, the E.M.F. of self- 
induction, so that E 1 = — E s ; but 

F T dI 

so 

1 dt 

The second part, E r , is that which is necessary to send 
current through the resistance of the circuit, according to 
Ohm's law, so that 

E r =RI. 

The impressed electromotive force, being the sum of E 1 
and E r , is therefore 

E=RI + L d i; 

at 



whence 



dt =L_. I= _L -w 



E-RI R E- RI 



MAGNETIC LAWS AND FACTS. 



27 



Integrating from the initial conditions t = o, I 
condition t = t y I = I', 
L 



o, to any 



R 



[log e (E-RI>)-log e E]. 



Therefore 



Rt 
L 



log e (- 



-RI' \ 

e r 



from which the instantaneous current value is 



r 



where e is the base of the natural system of logarithms 
and numerically equal to 2.7183. This equation shows 
that the rise of current in an inductive circuit follows a 
logarithmic curve, and that, when t is of sufficient magni- 
tude to render the second term negligible, the value of the 
current will be as given by Ohm's law, a condition which 
agrees with experimental ob- 
servations. 

A curve of the growth of 
current in a circuit having 
resistance and inductance is 
shown in Fig. 7, the values 
of I being calculated for the 
conditions noted. 

The natural logarithms used 
in preceding formulae can be obtained by multiplying the 
common logarithm of the number, the mantissa and charac- 
teristic being included, by 2.3026. 

19. Decay of Current in an Inductive Circuit. — If a 
current be flowing in a circuit having inductance and re- 
sistance, and the supply of E.M.F. be discontinued, with- 





<-<%c^) 












■ ^" 


* 


GROWING CURRENT 
(DIRECT) E.M.F. = 100 


{— 1 — 1 — h- 


-+ 


1 1- 


R = 10 
L = .2 

— I 1 1 r 



.02 .03 .04 .05 .06 .07 
SECONDS 



Fig. 7. 



28 DYNAMO ELECTRIC MACHINERY. 

out, however, interrupting the continuity of the circuit, the 
flow of current will not cease instantly, but the E.M.F.. of 
self-induction will keep it flowing for a time, with values 
decreasing according to a logarithmic law. 

An expression for the value of this current at any time, 
t seconds, after withdrawing the impressed E.M.F., may be 
obtained as in the foregoing section. The current at the 
instant of interruption of the impressed E.M.F. is due 
solely to the electromotive force of self-induction and may 
E 



o, 



E 

By integrating from the initial conditions t = o, I = — ' to 

R 

any condition t=t,I = I' i the instantaneous value of the 
current is found to be 

E -— 

which is the term that had to be subtracted in the formula 

for the growth of current. 



be 


represented by 


r' 


Therefore 






E = 


RI + L 


dl__ 

dt 


wh 


ence 


dt 


L 
R 


dl 
I 



DECAYING CURRENT 

E.M.F. = 

R= 10 

L= .2 

= 10 




This shows clearly that, while 
self-induction prevents the in- 
stantaneous attainment of the 
ultimate value of the current, 
there is eventually no loss of 
energy, since what is sub- 
tracted from the growing cur- 
rent is given back to the decaying current. 

Fig. 8 is the curve of decay of current in the same cir- 



MAGNETIC LAWS AND FACTS. 29 

cuit as was considered in Fig. 7. The ordinates of the 
one figure are seen to be complementary to those of the 
other. 

20. Quantity of Electricity Traversing a Circuit Due to 
a Change of Flux Linked with it. — In many dynamo 
tests, and in many magnetic investigations, it is necessary 
to measure, generally by means of a ballistic galvanometer, 
the quantity of electricity traversing a circuit due to a 
change of flux linked with it. If the circuit have a resist- 
ance of r and in dt time the flux linked with n turns 
changes by d<£>, then the instantaneous current 

7id<& 
dt 



But the quantity of electricity flowing during the time dt is 
dq = idt, hence 

nd<& 

dq = , 

r 

which is independent of time. So if the flux change from 
^ to $ 2 , then 

a = — - 1 n. 



If the resistance of the circuit be expressed in ohms and 
the flux in maxwells, the quantity of electricity in micro- 
coulombs will be 

n~— ■ ** - *i 

^ 100 R 

21. Work Performed by a Conductor Carrying a Current 
and Moving in a Magnetic Field. — Let a conductor carry- 
ing a constant current i be moved in a direction perpen- 



30 DYNAMO ELECTRIC MACHINERY. 

dicular to itself and to the lines of force of a magnetic field. 
Suppose it to move for dt seconds, and in that time to cut 
d<& lines of force. Then the induced E.M.F. e will be 

d& 

. The quantity of electricity dq that has to traverse 

the circuit against this E.M.F. during the time dt will 
be idt. Since potential is a measure of work, the work 
required to carry dq units of electricity against a difference 
of potential e is edq ergs. Hence the work in ergs, 

dw = edq — idt X — = id&. 
dt 

Therefore the current i, in cutting <£> lines of force, per- 
forms the work 

w = /<£ ergs. 

From this it is seen that the work done by a conductor 
carrying a current and cutting lines of force is independent 
of the time it takes to cut them. 

In the above discussion, if the field be non-uniform or 
the motion be irregular, the value of e will not be the 
same for each instant of time. But since the result 
obtained is independent of time, it is immaterial how the 
lines are arranged, and how the rate of cutting varies. 

22. Force Exerted between a Field and a Conductor 
Carrying a Current. — When a conductor moves in a field 
perpendicular to itself and to the lines of force, then, from 
the foregoing article, the work performed is 

w = i$ ergs. 

If the conductor be / centimeters long, and traverses a 
distance of d centimeters through a uniform field having a 



MAGNETIC LAWS AND FACTS. 31 

flux density of (B gausses, then the total number of lines 
of force cut is 

$ = ld($>, 
and the 

Work = Ud(8> ergs. 

But 

Work = force X distance = Fd. 

Il(& , 

.:F = il(8> = dynes. 

10 

This force acts perpendicularly to the wire and to the lines 
of force. 

23. Magnetomotive Force of a Circular Circuit Carrying 
a Current. — A thin circular conductor carrying a current 
forms a magnetic shell. The current produces magnetic 
lines of force each of which is closed upon itself and is 
linked with the conducting circuit. If a unit magnet pole 
be carried along any closed path linked with this conductor, 
coming back to point of starting, it will be subjected to 
magnetic forces that will vary in magnitude as its position 
is changed. These forces may also not have the same 
direction as the path of movement at any instant. But 
the sum of the products of the lengths of the path elements 
into the corresponding portions of the force, resolved along 
the path, will represent the difference of magnetic potential 
set up by the magnetic shell. This sum is the same as 
the product of the average force by the length of the path 
or the total work performed upon the unit pole. 

It is immaterial whether the pole be carried from one 
side of the shell to the other, Fig. 9, or the shell be turned 
bottom side up around the pole. In the latter case it is 




32 DYNAMO ELECTRIC MACHINERY. 

clear that all the lines emanating from the pole will be cut 
once, and once only, by the conductor, wherefore 4 n lines 

will have been cut. 

If current i flows in the con- 
ductor, then, by § 21, the work 

in ergs is 
» / 

^It y w = i$ = 4 ni. 

^/l\P"~~~S^' 

If there be n turns of the con- 
Fig. 9. 

ductor, each line of force will be 
cut n times, and the work will be 4 nni ergs. Hence the 
magnetic difference of potential between the two sides 

of a thin magnetic shell is 4 rjii or — Since — = 

10 10 

1.257, and since the current and the number of turns are 
usually regarded as a single quantity, namely ampere-turns, 
it follows that the difference of magnetic potential is equal 
to 1.257 times the number of ampere-turns. 

Difference of magnetic potential is a measure of the 
ability of the shell to set up lines of force, or, in other 
words, is a measure of its magnetomotive force. Magneto- 
motive force and difference of magnetic potential are simi- 
lar, just as are electromotive force and difference of electrical 
potential. Therefore magnetomotive force may be ex- 
pressed as 

M.M.F. = 1.257 ni. 

The practical unit of M.M.F. is the gilbert, so that a gil- 
bert = 1.257 X ampere-turn. 

24. The Toroid. — A uniform toroidal winding upon a 
ring-shaped iron core and carrying a current i produces the 
same flux density at all points on the axis of the toroid. 
Assuming the core to be removed and the portion of the 



MAGNETIC LAWS AND FACTS. 33 

flux which is not due to the iron to have the same distribu- 
tion as the flux occasioned by iron, then, if there be n turns 
in the coil, and the length of the axis be / cm., the work 
necessary to be exerted upon a unit pole to carry it once 
along the axis would be 4 izni ergs, which is also equal to 
the product of field intensity 5C at the axis into its length /. 
Hence the magnetizing force, that is, the strength or inten- 
sity of field, in the toroid is 

10I 
25. Magnetization Curves. — The permeability of air is 
constant for all magnetizing forces ; but this is not true of 
iron and other substances having permeabilities noticeably 
greater than unity. The values of /* for these paramag- 
netic substances depend also upon the chemical composition 
and physical condition of the latter. Values of magnetizing 
force, 5C, and the corresponding flux density, (B, in average 
commercial wrought iron, in cast iron, and in cast steel are 
given in the following table. The change of flux density 
with variation of magnetizing force is best shown by curves, 
called magnetization, or (B-3C, curves. In Figs. 10, 1 1, and 
12 are given the magnetization curves respectively of 
wrought and sheet iron, cast iron, and cast steel, the curves 
being plotted from the tabulated values. From an inspection 
of these curves, it is seen that an increase in magnetizing 
force results at first in a marked increase of flux per unit 
area, but as the material becomes magnetized further the 
increment of flux density lessens and finally approaches" a 
value proportional to the corresponding increment of mag- 
netizing force. At this point the material is said to be 
saturated, that is, an increase of M.M.F. shows no increase 
of flux density due to the presence of the given material. 



34 



DYNAMO ELECTRIC MACHINERY. 



WROUGHT AND SHEET IRON 



129 


20000" 


























































116 


18000- 


- V 






























































K 


[s 




















































iog 


16000 








\ 




^ 


^ 






















































> 




S 
















































rtX 






/ 






















































05 




/ 


















IS& 


>e 


































in 


12000 


/ 






















Av 


































/ 
























































bo 




1 
























































■i 








































\ 




















W 








































s 


\ 




























































\ 


















3S 


6000 
















































1 


















































/ 








I 
















































/ 




















































































1 


























































/ 


l 





o 


1 




n 


4 





5 





r 


i 


— - 


1 


I 





£ 


1) 


IOC 


110 


120 













Hi per centimeter 16 



JC 



fflX per inch 



Permeabilit y ^ 



Fig. io. 



CAST IRON 



65 


10000 
9000 
8000 
1000 

(B 

6000 
5000 
4000 
3000 








s 




























































\ 






























































\ 
































































\ 
































































\ 
































































V 


s 












































•& 














/ 




\ 

























































/ 








\ 










































03 










/ 












\^. 






































C< 








/ 
















\* 


«f 




































►. 32.5 






1 


' 




















f 








































/ 
























































J 2 ° 






/ 


























\ 






























■2 


































s 
































/ 
































S 


s 




























/ 




































\ 
























1000 










































\ 






















J 








































; 




















6.5 


7 




































^ 


y 
























T 






























































c 


10 20 30 40 50 60 70 80 9C 100 110 120 130 140 15 


n r per cen 


tiineter 16 32 48 64 80 96 112 


nl per inch 40 80 120 160 200 210 280 


PermeabihtyjtZ 100 200 300 400 500 



Fig. ii. 



MAGNETIC LAWS AND FACTS, 



35 



CAST STEEL 



139 


20000 

18000 
16000 
14000 

(B 

12000 
10000 
8000 
6000 
4000 
2000 




1 














I 






| 
























































































































































103 




































































s 


s 
































































\ 


v 
























































































































S 




















































s ,0 








/ 






















^ 
































Z 05 






z 
























'^Tfc 




























/ 


























































1 52 




/ 




























































/ 




















R£ 


SID 


UA 


-f> 


AG 


NE 


TIS 
















* 








5 














































<" 


























































/ 




















































26 






















































































































































































































































































































J 


> 10 15 20 25 i 





5 4) 


a 


o /:.., •: ; 


•W- J per centimeter 8 16 24 32 40 43 56 


n I per inch 20 40 60 80 100 120 140 


Permeability p. 1000 2000 



=JC 



Fig. i2. 



DATA FOR. (B-X CURVES. 

AVERAGE FIRST-QUALITY METAL. 



X 


AMPERE- 
TURNS 


AMPERE- 
TURNS 


WROUGHT AND 
SHEET IRON. 


CAST IRON. 


CAST STEEL. 
















PER INCH 




KILO- 




KILO- 




KILO- 




TIMETER 
LENGTH. 


LENGTH. 


(B 


LINES 
PER 

SQ. IN . 


<B 


LINES 

PER 

SQ. IN. 


CB 


LINES 

PER 

SQ. IN. 


10 


7-95 


20.2 


1 1 800 


74 


3900 


25.2 


12000 


77 


20 


I5-90 


40.4 


14000 


90 


55oo 


35-5 


13800 


89 


30 


23-85 


60.6 


15200 


98 


6500 


42.0 


14600 


94 


40 


3I.80 


80.8 


15800 


102 


7100 


45-7 


15400 


99 


50 


39-75 


IOI.O 


16400 


106 


7700 


49-5 


16000 


103 


60 


47.7o 


121. 2 


16800 


108 


8200 


53-o 


16400 


106 


80 


63-65 


I6I.6 


17200 


in 


8900 


57-2 


16700 


108 


IOO 


79.5o 


202.0 


17600 


114 


9300 


60.0 


17600 


113 


125 


99.70 


252-5 


17800 


us 


9700 


62.4 


18200 


117 


I50 


119.25 


303-0 


18000 


116 


IOIOO 


65.8 


18600 


120 



3C = 1-257 (*/per cm.) = .495 («/per in.), (ft = .155 (<£ per sq. in.) 



36 DYNAMO ELECTRIC MACHINERY. 

Curves of permeability, plotted in terms of flux density, 
for wrought iron, cast iron, and cast steel are also shown 
in the accompanying figures. 

In general all substances mixed with or alloyed with iron 
lower its permeability. In steel and cast iron the permea- 
bility seems to be in inverse proportion to the amount of 
carbon present. Carbon in the graphitic (not combined) 
form lowers the permeability less than carbon when com- 
bined. In cast iron and cast steel such substances as tend 
to give softness and greater homogeneity to the metal 
when present in limited amounts, say 2 per cent, increase 
the value of /i. Aluminum and silicon act in this way. 

The physical condition of the metal also affects its per- 
meability. Chilling in the mold, when casting, lowers it, 
as does tempering, or hardening the metal by working it. 
On the other hand, annealing increases the permeability. 

A piece of iron or steel, subjected to a small magnetizing 
force, has its permeability increased by increasing the tem- 
perature until a critical temperature is reached, when it 
falls off rapidly to almost unity. For stronger magnetiza- 
tion the permeability does not rise so high at the critical 
temperature, and does not fall off so sharply after it. The 
value of this critical temperature lies between 650 C. and 
900 C, depending on the test piece. The influence of 
temperature upon permeability is very small for the changes 
in temperature occurring in practical operation, and is 
therefore negligible. 

26. Reluctance and Permeance. — In the flow of mag- 
netic lines of force the reciprocal of the permeability, — , is 

called the reluctivity. The total reluctance, tending to 
oppose the passage of magnetic lines under the influence 



MAGNETIC LAWS AND FACTS. 37 

of a magnetic difference of potential, is directly as the 
length and the reluctivity of the medium and inversely as 
its cross-section. Hence the total magnetic resistance or 

reluctance = - — : — reluctivity. 

cross-section 

Reluctivity is usually represented by v I = -J. Hence for 

a medium of cross-section A square centimeters and length 
/ centimeters, the reluctance 

(R= -v. 
A 

The unit in which (R is expressed is called the oersted. 

Permeance is the reciprocal of the reluctance, hence the 
permeance 

^ A A 

It must be remembered that y and t u are not constant for 
some substances, but depend for their values upon the 
strength of the magnetizing force 3C which is acting upon 
the substances. 

27. Relation Between Magnetomotive Force, Magnetic 
Flux, and Reluctance. — The flux produced in a magnetic 
circuit by a magnetomotive force may be expressed by an 
equation similar to that expressing the current flow in an 
electric circuit due to an impressed electromotive force, 
which is 

electromotive force 

current = : 

resistance 

The corresponding equation for the magnetic circuit is 

. n magnetomotive force 

magnetic nux = — , 

reluctance 



38 DYNAMO ELECTRIC MACHINERY. 

or symbolically / 

^ _ M.M.F. _ Ann To 

liA 

Since the unit of magnetic flux is one line of force or the 

maxwell, the unit of magnetomotive force is the gilbert, 

and the unit of reluctance is the oersted, this equation may 

be written 

„ gilberts 

maxwells = 

oersteds 

The application of this equation is not as simple as 
that of the corresponding equation of the electric circuit. 
Electric circuits, in general, exist in media of zero electric 
conductivity, and therefore permit of accurate calculations, 
since the leakage is inappreciable. Magnetic circuits, on 
the other hand, are situated in media which have permea- 
bilities of at least unity and hence much leakage is present, 
and precise calculations require a consideration of all flux 
paths. In the designing of dynamo electric machinery, 
however, one or more paths of low reluctance are presented 
to the magnetizing force, and these are so shaped that the 
leakage paths offer a comparatively high reluctance. 

28. Hysteresis. — If a piece of iron become magnetized, 
and the magnetizing force be then removed, the iron does 
not become completely demagnetized. A certain magnet- 
izing force in the opposite direction must be applied to 
bring it back to its original condition. This phenomenon, 
where " changes of magnetism lag behind the changes of 
force," has been termed hysteresis. Because of hysteresis 
a (B-3C curve taken with continuously increasing values of 3C 
to the maximum and then with continuously decreasing 



MAGNETIC LAWS AND FACTS. 



39 



values of JC to a negative maximum, and soon, will assume 
the shape shown in Fig. 13. The distance OA represents 




Fig. 13. 

the coercivity, that is, the magnetizing force necessary to 
bring the iron from a magnetic to a neutral state. The 
distance OC represents the retentivity, that is, the value 
of the residual magnetic flux density in the iron after the 
magnetizing force has been removed. 

The area inclosed by the curve represents the energy 
lost in carrying the iron through one cycle, i.e. from a 
maximum magnetization in one direction to a maximum in 
the opposite direction and back to the original condition. 
Suppose the magnetization to be due to a current / flowing 
in a solenoid of n turns. If, in a short interval of time dt, 



40 DYNAMO ELECTRIC MACHINERY. 

a change of d$> be made in the flux which is linked with 
the solenoid, then this change will induce an E.M.F. in the 
solenoid, which, during the interval of time dt, will be 
equal to 

nd$ 



io 8 dt 



volts. 



During this time work must be performed to maintain this 
current /, and its magnitude is 

Eldt = > 

io 8 

for Idt represents the quantity of electricity which is trans- 
ferred from one point to another, between which there ex- 
ists a difference of potential E. Now <£ =A(& (§ 14) and 

10 1CZ 
hence d<$ = Ad($>. Furthermore, nl = (§ 24). Hence 

47T 

the work during the time dt is 

Al 
Eldt = — — 3Cd(B joules. 
io 7 4- 

Supposing the magnetizing force to vary cyclically, taking 
t seconds to make one cycle, then the work per cycle is 

ai r~^~ ® m 

Elt = — - — / 3Cd($> joules. 

If the number of cycles completed in one second be/, then 
/ = — 1 and the power in joules per second, that is, the power 
in watts, equals 

fAl r + ® m to- 7 /» + ®»» 

EI = -^- / 3Cd(B = —fv / Kd®» 

lO^nJ _ (S>m 4- J .^ 

where v is the volume of iron in cubic centimeters. The 



MAGNETIC LAWS AND FACTS. 41 

integral expression is evidently the area contained by the 
hysteresis loop. 

The value of this integral is dependent upon (B m , upon 
the retentivity of the kind of iron, and upon its coercivity. 
Steinmetz has shown that for all practical purposes the 
value of the integral for flux densities ranging from 2000 
to 14000 gausses may be expressed by the empirical formula 



4 






where 77 is a constant depending upon the physical and 
chemical properties of the iron. Therefore the power lost 
in watts due to hysteresis may be written 

P„= 1 o-' */,««. 

Values of the constant 7? are given in the following table : — 

HYSTERETIC CONSTANTS. 

Best soft iron or steel sheets 0.00 1 

Good soft iron sheets 0.002 

Ordinary soft iron 0.003 

Soft annealed cast steel 0.008 

Cast steel 0.012 

Cast iron 0016 

Hard cast steel 0.025 

The hysteretic constant increases with continued heating, 
and this effect is called ageing. Annealing, while it in- 
creases the permeability, also increases the hysteretic con- 
stant as well as the ageing effect. 

The magnitude of the hysteretic constant is largely de- 
pendent upon the mechanical structure of the iron. To 
attain the smallest value, the iron should not be of homoge- 
neous structure, but should be more compact in directions 
perpendicular to the direction of the flux than in transverse 
directions 



42 DYNAMO ELECTRIC MACHINERY. 

29. Eddy Currents. — When a mass of iron is subjected 
to a pulsating flux, electromotive forces are set up in the 
iron which produce currents therein, called eddy, or Fou- 
cault, currents. The flow of these currents represents an 
expenditure of energy appearing as heat. In order to 
prevent excessive heating of such portions of dynamo 
electric machinery subject to rapid reversals or changes of 
magnetization, these portions are constructed of laminated 
iron, the laminae being transverse to the direction of flow 
of the eddy currents, but longitudinal with the magnetic 
flux. Each lamina is more or less thoroughly insulated 
from its neighbors by the natural oxide on the surface or 
by Japan lacquer. 

The loss of power due to eddy currents could be made 
inappreciable by the use of laminae sufficiently thin ; but a 
limitation exists due to the decrease in effective iron cross- 
section caused by the waste of space which is taken up by 
the insulation between adjacent laminae. The thickness 
of the laminae generally used for dynamo armatures is 
between 0.014 and 0.02 inch, and the space between 
laminae is usually somewhat less than 0.002 inch. 

A formula for the calculation of the power, in watts, 
lost in iron due to eddy currents, based upon the assump- 
tion that the laminae are perfectly insulated from each 
other, is 

p e = kvf 2 m 2 m , 

where k = a constant depending upon the resistivity of 
the iron, its value being about 1.6 X io _u , 
v = volume of iron in cubic centimeters, 
/ = thickness of one lamina in centimeters, 
/ = number of magnetic cycles per second, 

and (B m = maximum flux density (i.e. <£ m per sq. cm.). 



PROBLEMS. 43 

The armatures of dynamos are usually provided with 
projecting teeth, and therefore the flux density between 
an armature and its field poles is greatest opposite the 
teeth and is a minimum opposite the slots. As the arma- 
ture rotates, this variation of flux produces to some extent 
eddy currents in the pole faces. To reduce the loss occa- 
sioned thereby, the pole faces are sometimes also constructed 
of laminated iron. 

PROBLEMS. 

i. Two cylindrical magnets, 1.8 cm. in diameter, are mag- 
netized to an intensity of 500 units pole for each square cm. of 
cross-sectional area, and their north poles are placed 8 cm. 
apart. Compute the force of repulsion between the two north 
poles. 

2. What is the total flux from each pole of the magnets 
specified in Prob. 1, considering the poles to be isolated and 
concentrated at points ? 

3. A conductor 24 inches long, moving parallel to itself and 
at right angles to a magnetic field having an intensity of 40000 
maxwells per sq. in., traverses 5 ft. in 3.5 seconds. Determine 
the average E.M.F. in volts induced in the conductor during 
this interval. 

4. In 2 l F second the current strength in a circuit, having an 
inductance of 0.6 henry, falls from 30 to 15 amperes. What is 
the magnitude and direction of the average induced E.M.F. in 
the circuit due to this change of current ? 

5. What is the inductance of a circuit having 5 ohms resist- 
ance and in which the instantaneous value of the current 0.03 
second after impressing no volts upon the circuit is 13.9 
amperes ? 

6. What would be the current in a circuit, having 10 ohms 
resistance and 0.3 henry inductance, 0.02 second after suppress- 
ing the initial E.M.F. of 80 volts ? 



44 DYNAMO ELECTRIC MACHINERY. 

7. The force exerted on a wire 5 ft. long, which carries a 
current of 50 amperes, is 1800 dynes. What is the intensity of 
the magnetic field in which this wire is situated ? 

8. A brass toroid, having a mean diameter of 20 cm., is 
completely wound with 8 turns of wire per cm. of axial length. 
Determine the magnetic field intensity at the axis, when a cur- 
rent of 50 amperes flows through the winding. Calculate the 
total magnetomotive force which sets up this field. 

9. A wrought-iron toroid, wound with 20 turns of wire per 
inch of axial length, has a current of 3.5 amperes flowing 
through the winding. Determine the flux density and perme- 
ability of the iron from the data given in § 25. 

10. The flux density in a cylindrical cast-iron rod 5 cm. in 
diameter and 30 cm. long is 6000 gausses (^1= 250). Compute 
the reluctance and permeance of the rod between the two faces. 

1 1. Calculate the total number of ampere-turns necessary to 
produce a flux density of 6000 gausses in the toroid of Prob. 10. 

12. A closed core, composed of the best steel sheets 
0.035 cm - thick, has a volume of 5400 cu. cm., and is subjected 
to 100 magnetic reversals per second, i.e. f= 50. Calculate 
the hysteresis and eddy current losses when the maximum flux 
density in the core is 3500 gausses. 



ARMATURES. 45 



CHAPTER III. 

ARMATURES. 

30. Dynamos. — Dynamos may be denned as machines 
to convert mechanical energy into electrical energy, or 
electrical energy into mechanical energy, by utilizing the 
principle of electromagnetic induction. A dynamo is known 
as a generator when mechanical energy, supplied in the 
form of rotation, in all commercial machines, is converted 
into electrical energy, which may be delivered either as 
il direct current" or as " alternating current." When the 
conversion of energy takes place in the reverse order, the 
dynamo is called a motor. 

31. Principle of Action of a Generator. — If a loop of 
wire be revolved in a magnetic field about an axis perpen- 
dicular to the lines of force, as in Fig. 14, then each side 
(but not the ends) of the loop is a conductor moving across 
the lines of a magnetic field, and as such will have an 
E.M.F. induced in it. Since the motion of one conductor 
is up while that of the other is down, the directions of the 
induced E.M.R's in the two sides would be opposite to 
each other, but since they are on opposite sides of a loop, 
the pressure will be cumulative; i. e. instead of neutralizing 
each other, the two pressures will be added to each other. 
If now the two ends of the wire from which the loop is 
made be respectively connected with slip rings, and a cir- 
cuit be completed through contacts sliding on them; a cur- 
rent will flow. When the loop, in its revolution, reaches a 



4 6 



DYNAMO ELECTRIC MACHINERY 



position (as illustrated in Fig. 14) such that the conductor 
that was previously moving upward begins to move down- 
ward, then the direction of the induced E.M.F. will be 
changed in both sides of the loop, and the direction of the 




Fig. 14. 

current through the circuit will be changed. For each 
complete revolution the current changes direction twice. 
It is an alternating current, and the supposed machine is 
an alternating-current generator, or simply an alternator. 

32. The Function of the Commutator. — If, instead of 
connecting the two ends of a loop of wire revolving in a 
magnetic field to slip rings, they be attached one to each 
half of a split metal ring mounted on the same shaft, the 
two halves being insulated from each other, and brushes be 
provided, which are so placed that at the instant the induced 
E.M.F. in the loop changes in direction the brushes will 
slide across from one of the halves to the other, then the 
current, while reversed in the loop, will flow in the same 
direction in the external circuit. This arrangement, called 



ARMATURES. 



47 




a commutator, is employed when it is desired to obtain a recti- 
fied, continuous or direct current. A dynamo so equipped 
is called a direct-current generator, or simply a generator. 

If the loop were wound double, i.e. have four conductors, 
before the ends were attached to commutator segments, 
and if the speed and the strength of the magnetic field be 
maintained constant, twice the E.M.F. will be produced. 

For a single loop, the commutator would consist of two 
cylindrical pieces or segments, as shown in Fig. 15. In 
this case there would be no 
E.M.F. produced at the in- 
stants when the brushes pass 
from one segment to the 
other, and hence the current 
would fall to zero twice during 
every revolution of the loop. 
If two loops, placed at right Flg - I5# 

angles to each other, are rotated in a magnetic field, one 
or the other would always be cutting lines of force and at 
no time could the pressure be zero. To satisfactorily collect 

current from this arrange- 
ment requires four commu- 
tator segments and a system 
of connections similar to that 
shown in Fig. 16. In this 
case the E.M.F. would fluc- 
tuate, but not so badly as in 
the previous one. If the num- 
ber of loops be increased and 
Fig. 16. the number of commutator 

segments be correspondingly increased, the E.M.F. fluctua- 
tion of such an arrangement will become practically negligible. 




48 DYNAMO ELECTRIC MACHINERY. 

33. Electromotive Force Generated. — The magnitude of 
the electromotive force induced in a conductor of length 
/cm. moving parallel to itself with a velocity of v cm. per 
sec. across a uniform magnetic field having a flux density of 
(B gausses is 

E = ($>lv io~ 8 sin a volts, § 15 

where a is the angle between the paths of the flux and of 
the conductor. If a single loop (two conductors) revolve 
about an axis that is in the plane of the loop and perpen- 
dicular to the flux with an angular velocity of ^revolutions 
per minute, the linear velocity of the conductors will be 

V 

v = 2 nr -r-i 
60 

where r is the distance in cm. of each conductor from the 
axis. The instantaneous E.M.F. for a loop of s conductors 
is therefore 

E' = 2 x&lrs • t-io" 8 sin a volts. 
60 

But 2 r/(R is the maximum flux passing through the loop, 
$ m ; hence 

E' = 7t$ m s— io -8 sin a volts. (1) 

The maximum value of the induced E.M.F. for the loop 
of s conductors is attained when a = 90 , i.e. when the 
conductors move perpendicularly across the magnetic flux. 
This maximum value is 

E m = 7z<& m s — io- 8 volts. (2) 

The instantaneous E.M.F. is therefore 
E' = E m sin a, 



ARMATURES. 



49 



which is the equation of a sine curve. Thus the sine curve, 
Fig. 17, shows the instantaneous values of the induced elec- 
tromotive force as the angle a varies from o° to 360 . 




Fig. 17. 



The average E.M.F. during a half revolution is obtained 
by dividing the area of one lobe by the base line. Thus 



E a v = 



£ 



E m sin ada 



En, 

71 



[- cosa]"=-£, 



(3) 



Substituting the value of E m , there results the average 
E.M.F. induced in the loop of s conductors 

V 



2 $ m s — io~ 8 volts. 



(4) 



If this loop be provided with a commutator as explained 
in the foregoing section, 
the direction of the vol- 
tage impressed on the 
external circuit remains 
the same, but the magni- 
tude varies sinusoidally 
as shown in Fig. 18. 

If a similar loop be 




360 



90 180 270 

a 
Fig. 18. 

placed 90 from the other, the magnitude of the induced 



So 



DYNAMO ELECTRIC MACHINERY. 



E. M. F. therein will be the same as that in the first, but 
corresponding instantaneous values in the two loops occur 

90 apart, as shown in 
Fig. 19. The resulting 
electromotive force at any 
instant may be found by 
adding the E.M.F.'s in- 
duced in the two loops at 
that instant. Thus in 
Fig. 19 the dotted line 
shows the resulting pres- 
sure for two loops situated 90 apart. 

The instantaneous value of the electromotive force in- 
duced in the first loops is 




Fig. 19. 



E/ = E m sin a, 

and that induced in the second is 

E 2 '=E m sin ( 9 o° + a); 

therefore the resulting instantaneous value of voltage in- 
duced in the loops when connected in series is 

£' = £/ + E 2 ' = E m [sin a + cos a]. 

In general, if there be m loops connected in series and 

displaced — electrical degrees from each other, and rotating 
m 

about a common axis in a uniform magnetic field, then 
the instantaneous pressure will be 

E'-E m [sin a + sin(^+a)+.. + sm(*^+«)],(5) 



ARMATURES. 5 1 

and the average pressure will be 

( n — + a ) \ da ; 

\ m /J 



2 m 
-f sin 
whence 



E av = —*■ = 2 M$ m s • — io- 8 volts. (6) 

The total number of conductors 5, connected in series 
and in circuit between two brushes, is equal to the product 
of the number of loops m and the number of conductors 
per loop, s ; or 

6* = ms. 

By substituting this value in (6), there is between brushes 

£av = 2$„^I0- 8 V0ltS. ( 7 ) 

The maximum and minimum values of instantaneous pres- 
sures are obtained respectively by substituting for a in (5) 

the quantities — and o. The resulting expressions are 
2 m 

4ax = Em CSC ~^~ 

2 m 
and 

£min = Em COt T^ ' 

2 m 
where E m is the maximum E.M.F. per loop. The percent- 



52 DYNAMO ELECTRIC MACHINERY. 

age fluctuation of the E.M.F., therefore, can be represented 
by 

E av m \_ 2 m 2 m J 

Thus, for twelve loops revolving in a bipolar field, the 
fluctuation of electromotive force is 1.7 per cent. 

In order to render the foregoing formulas applicable 
to a multipolar field, it is necessary to insert the symbol 
p, which denotes the number of pairs of field poles. The 

product of the terms — and / represents the number of 
60 

magnetic cycles some of the iron of the machine passes 

through in one second. It is termed frequency, as in 

alternating-current work, and is represented by/*. Thus 

Therefore the average electromotive force available between 
brushes is 

£av = 2<I> m S/lO- 8 VOltS, (9) 

where 3> m is the total flux per pole passing through the 
armature. 

34. The Armature. — In a dynamo, the loops of wire in 
which E.M.F. is induced by movement in a magnetic field, 
together with the iron core that sustains them, with the 
necessary insulation, and with the parts connected imme- 
diately thereto, constitute the armature of a dynamo. An 
armature in which both sides of the loop of wire cut lines 
of force, as in the cases just described, is called a Drum 
Armature. A kind of armature less generally used is the 




Fig. 20. 



ARMATURES. 53 

Ring Armature, illustrated diagrammatically in Fig. 20. 
Here the lines of force emanating from the N. pole flow 
through the iron core of 
the ring, and very few 
across the air space inside 
the ring. Hence, when 
wires are wound on the 
ring, and the whole is 
revolved about an axis per- 
pendicular to the plane of 
the ring, only the wires on the outside face of the ring 
cut lines of force, those on the inside serving only to com- 
plete the electrical circuit. 

Drum armatures have all the conductors on the peripheral 
surface, and therefore have a greater percentage of active 
wire than ring armatures. Drum armature cores are 
very often constructed in the form of a ring, because 
of better ventilation and economy of iron; and such 
construction should not be confused with that of ring 
armatures. 

A drum armature of large diameter and of short length 
in the axial direction may have more wire exposed on its 
ends than on its periphery. The pole pieces are some- 
times placed at the ends, and the armature is then called 
a Disk Armature. This type is seldom used in this 
country. 

35. The Field Magnets. — Almost all dynamos have their 
magnetic fields produced by electro-magnets, and these are 
called field magnets. In small machines the field magnets 
are usually bipolar, i.e. they have one North and one South 
pole, with the armature revolving between them. Bipolar 
machines are made in many forms, a few of which are 



54 



DYNAMO ELECTRIC MACHINERY. 



shown in Figs. 21, 22, and 23. The magnetizing coils, or 
field coils, may be placed on both legs of the magnet, on 





Fig. 21. 



Fig. 22. 



one leg, or on Wit yoke which connects the two. The best 
and most used bipolar arrangement is the enclosed type of 




Fig. 23, for the coils are almost completely surrounded by 
iron, thereby securing protection from mechanical injury, 
as well as avoiding excessive magnetic leakage. 



ARMATURES. 



55 



A typical form of multipolar field magnet structure is 
shown in Fig. 24, in which an even number of poles are so 
wound as to be alternately magnetized North and South. 




Fig. a 4 . 



36. Armature Windings. — It is possible to connect the 
conductors of an armature to each other and to the com- 
mutator segments in a great many ways that will permit of 
satisfactory operation. The design of an operative scheme 
of winding is, to a great extent, a geometrical problem. Of 
the many possible and proposed schemes of winding, those 
which have been adopted and are still used in the construc- 
tion of standard machines are characterized by economy 
of copper, by good time constants to secure satisfactory 
commutation, and by such coil shapes as permit of con- 
venience in construction and assembling and of accessibility 
for making repairs. 

Direct-current armature windings are divided into two 



56 DYNAMO ELECTRIC MACHINERY. 

classes, namely open-coil and closed-coil windings. The 
former are used almost exclusively on series constant-cur- 
rent machines, such as the Thomson-Houston arc-light 
generators, and will be discussed in a later chapter. With 
open-coil windings, only those conductors which are con- 
ductively connected between the commutator segments, 
which the brushes are momentarily resting on, are effective 
in supplying an electromotive force. 

Closed-coil windings are much more generally used. If 
the wire of an ordinary, that is, single, closed-coil winding 
were removed from the armature and uncoiled, it would 
form a closed loop, and the points of connection with the 
commutator segments would be equidistant from each other. 
Some closed-coil windings are so constructed that the wire, 
if removed from the armature core and uncoiled, would form 
two or more closed endless loops. Such windings are 
termed duplex, triplex, or quadruplex windings, according 
to the number of endless loops, whether two, three, or four. 
Such multiplex windings are sometimes employed on ma- 
chines of large current output ; but their use is relatively 
infrequent, and therefore, unless explicitly stated to the 
contrary, single, or simplex, windings will hereinafter be 
understood. 

It is convenient, in treating of armature windings, to call 
each of the portions which terminates at two commutator 
segments an element of the winding. An armature element 
may consist of one or more armature coils. Those parts 
of an armature element which lie on the periphery of the 
armature and in which E.M.F.'s are induced are called in- 
ductors. Thus, in the ring type of armature, only the por- 
tions of the wire on the outer surface of the ring constitute 
its inductors. A drum armature element, however, has two 



ARMATURES. 57 

inductors lying axially on the core surface. In the follow- 
ing discussion of armature windings, when one inductor is 
mentioned it does not imply that only one wire is meant ; 
further, an element said to be formed by two inductors may 
be a coil of many turns. Simplification of the winding 
diagrams is effected in this manner. 

Two types of closed-coil windings for direct -current arma- 
tures are to be distinguished, namely two- circuit or wave 
windings, and mnltiple-circuit or lap windings; the former 
being used principally on machines of small output, and the 
lap winding on machines of intermediate and large output. 
In the wave winding there are always two circuits between 
the brushes, regardless of the number of field poles on the 
machine, each of the circuits carrying one-half of the total 
current. For this type of winding only two brushes are 
required, but it is usual in practice to provide as many 
brushes as there are poles, in order to avoid excessive spark- 
ing at the commutator. There is a slight, but generally 
neglected, E.M.F. induced in the portions of the winding 
included at any instant between the different sets of brushes 
of the same polarity. It is quite small, however, as com- 
pared with the E.M.F. induced between brushes of opposite 
polarity. In the lap winding there are as many circuits 
between the brushes as there are field poles, and each cir- 
cuit carries — of the total current. For armatures of this 
2 p 

type as many brushes as field poles are required. 

Starting from a certain commutator segment and passing 
clockwise over / elements of a simplex wave winding, or 
one element of a simplex lap winding, one reaches the next 
adjacent segment either to the right or to the left. If it be 
to the right it is called a progressive winding ; and if to the 



58 



DYNAMO ELECTRIC MACHINERY. 



left, it is called a retrogressive winding. The use of the 

latter is somewhat more economical in copper. 

A 4-pole two- circuit, or wave winding is shown diagram- 

matically in Fig. 25, in which the inductors are represented 

by the short radial lines 
and the end connections by 
the lines joining them. 
The brushes are placed 
inside the commutator for 
clearness. Fig. 26 shows 
the same winding more 
clearly in developed form, 
the seventeen commutator 
segments being lettered 
s from a to q. Tracing the 

Fig. 25. method of interconnecting 

the 34 armature inductors, and starting from segment a, 





Fig. 26. 

it is seen that connection is made to inductor No. 10, 
which is connected at the rear to inductor No. 19; and 



ARMATURES. 



59 



this at the front is joined to segment j and also to inductor 
No. 28. Proceeding in this way, the winding scheme may- 
be tabulated as follows : 



REAR END 



FRONT END 



IO 
28 
12 

30 
14 

3 2 

16 

34 

18 

2 

20 

4 
22 

6 
24 

8 
26 



19 

3 

21 


i 

b 
k 


5 


c 


2 3 

7 


\ 


2 5 


m 


9 


e 


27 


n 


11 


f 


29 





13 


g 


3i 
'5 


P 
h 


33 


q 


17 


1 


1 


a 



28 

12 

30 
14 

l6 

34 

18 

2 

20 

4 
22 

6 
24 

8 
26 
10 



Thus the winding forms one closed circuit, and it is evident 
that there are two paths from the positive to the negative 
brush. It is a progressive winding. 

The number of inductors spanned by the end connections 
at each end is called the winding pitch, and is represented 
by X. In Fig. 26 the inductors are numbered consecutively 
from 1 to 34 in passing around the armature, and inductor 
No. 10 joins inductor No. 19 at the rear; the rear-end 
winding pitch is therefore 9, or 

X r = 19- 10 = 9. 

At the front, or commutator, end, inductor No. 19 joins in- 
ductor No. 28, so that the front-end winding pitch is also 

X f = 9. 

It occurs frequently, however, that the front and rear wind- 



6o 



DYNAMO ELECTRIC MACHINERY. 



ing pitches are different, in which case the mean winding 



pitch is 



X 



K + k 



There exists a definite relation between the total number 
of inductors on the surface of an operative armature, the 
number of pairs of poles, and the winding pitch. For the 
wave winding, the permissible total number of inductors is 

C= 2p\ ± 2. 

Hence, for the four-pole machine of Fig. 26 the total 
number of inductors would be 2 X 2 X 9 ± 2 ; that is, C = 
38 or 34. The latter number was here chosen. 




ARMATURES. 



6l 



A six-pole wave winding for a drum armature having 31 
slots is shown in Fig. 27. There are 62 inductors, the 
even and odd numbered ones representing the lower and 
upper inductors in the slots respectively. The winding 
pitches for this armature are 

K = 9> 
X f = 11, 
and X = 10. 

A six-pole, retrogressive, lap or multiple-circuit, in this 
case six-circuit, winding is shown diagrammatically in Fig. 
28, and it is seen that 
there are six paths in 
parallel from the positive 
to the negative brushes. 
There are 80 inductors, 
the winding pitches 
being 

X r = n 
and X f = 13. 
In the lap winding, the 
front and rear winding 
pitches cannot be equal 
and the difference be- Fig. 28. 

tween them must be some multiple of 2. If there be two 
inductors per slot, and the lower ones be even-numbered 
and the upper ones odd-numbered, as usual, then the front 
and rear winding pitches must be odd, and therefore the 
mean winding pitch, X, is ■ always an even number. The 
expression for the total number of inductors on a lap 
winding for p pairs of poles allows more latitude than is 
the case with wave windings, and is 

C = 2 pX. 




62 



DYNAMO ELECTRIC MACHINERY. 



Herefrom the mean winding pitch is 

but frequently values of X are taken which are considerably 
smaller than the value obtained from this formula. In the 

present case, for example, X = 12 instead of — . Such wind- 

6 
ings are called short-chord windings. 

As previously stated, a winding element of a drum arma- 
ture has two inductors, yet there may be any number of 
turns of wire in the element. This is often so in practice, 
where high terminal voltages are desired. Fig. 29 shows 





Fig. 29. 

a three-turn element for a wave winding, and Fig. 30 a 
similar element for a lap winding. Winding pitches are 
sometimes, but not in this text, expressed by the number 
of spanned core-slots or subtended angles at the axis, the 
latter being expressed in radians or degrees either electrical 
or mechanical. 

37. Multiplex Armature Windings. — In the simplex 
armature windings thus far considered, a winding would 



ARMATURES. 63 

consist of one complete circuit. In multiplex windings, on 
the other hand, there may be two or more distinct circuits 
completely insulated from each other, and each of these 
might be provided with a separate commutator and set of 
brushes. The usual practice, however, is to provide only 
one commutator with the segments pertaining to one wind- 
ing intermeshed with those belonging to the other wind- 
ings. Thus no greater number of brushes is required than 
for a corresponding simplex winding, yet they must be 
considerably wider so that simultaneous commutation of 
all the circuits may be going on. 

Starting from a certain commutator segment, and trav- 
ersing/ elements of a simplex wave winding, or any one 
element of a simplex lap winding, one reaches the next 
adjacent commutator segment. Proceeding in like manner 
with a duplex winding, one reaches the second following 
segment from the starting point ; and similarly, with a 
triplex winding, one reaches the third following segment, 
etc., the intermediate segments being connected to the 
other windings. Such multiplex windings, which consist 
of a number of complete and independent simplex wind- 
ings, are multiply re-entrant, that is, each individual circuit 
re-enters upon itself to form a closed circuit. Multiplex 
armature windings may be singly, doubly, triply, or in 
general, multiply re-e?itrant. 

A singly re-entrant winding is one in which, by succes- 
sive angular advances, the entire winding is traversed before 
returning to the starting point. A doubly re-entrant wind- 
ing is one in which only half of the winding is traversed 
before reaching the initial segment. Similarly, in a triply 
re-entrant winding only one-third thereof is traversed. The 
number of separate circuits on an armature determines the 



64 



DYNAMO ELECTRIC MACHINERY. 




degree of re-entrancy. The number of times it is necessary 
to pass around the armature in traversing a complete circuit 

must not be confounded 
with the degree of re- 
entrancy. 

Fig. 31 depicts a six- 
pole, retrogressive, two- 
circuit, singly re-entrant 
duplex winding compris- 
ing 58 inductors. A four- 
pole, retrogressive, two- 
circuit, triply re-entrant 
triplex winding, having 66 
inductors, is shown in 
Fig. 32. A duplex wind- 
ing may be either singly or doubly re-entrant, a triplex 
winding may be either singly or triply re-entrant, and a 
quadruplex winding may 
be singly, doubly, or quad- 
ruply re-entrant. Multi- 
plex windings beyond 
these are rarely used in 
practice. 

The general formula for 
multiplex wave windings 
is 

C = 2 pX ± 2 y, 

where C, p, and X have 

the same significance as 

before, and where y is the 

multiplicity of the winding, whether duplex, triplex, and so 

on. For a given multiplex winding, the choice of the 




ARMATURES. 6$ 

mean winding pitch, and therefore also the total number 
of inductors, depends upon the degree of re-entrancy 
desired. The highest common factor of X and y expresses 
the degree of re-entrancy. Thus, for the duplex winding 
of Fig. 31, y = 2 

and X = 9 ; 

therefore the winding must be singly re-entrant, since the 
highest common factor of 2 and 9 is 1. The total number 
of inductors for this winding might be 

C=2X3X9±(2X2) 
= 50 or 58. 

The latter value was here chosen. 
For the triplex winding of Fig. 32, 

y = 3 and ^=15; 

hence the winding is triply re-entrant. The total number 
of inductors might also have been 54 instead of 66 as 
shown. 

In multiplex lap windings, the degree of re-entrancy is 
equal to the highest common factor of half the number of 

C 

inductors,-, and the multiplicity of the winding, y. The 

mean winding pitch is chosen as near as possible to 

the same as for simplex lap windings. 

38. Equalizing Connections. — The electromotive forces 
generated in different sections of an armature are not ex- 
actly equal, due to inaccurate centering of the armature 
and to the unavoidable differences of distance of the arma- 
ture conductors from the field pole pieces. The E.M.F. 



66 DYNAMO ELECTRIC MACHINERY. 

differences, although small, nevertheless set up local cur- 
rents in the armature, and these may result in excessive 
heating of the conductors and sparking at the commutator. 
In wave windings, little difficulty is experienced in this 
respect, because the inductors are connected in series and 
are distributed under all the poles ; but in lap windings 
large internal currents are produced in the armature which 
cause operative troubles. To minimize troubles from these 
currents, lap-wound armatures of large generators are sup- 
plied with eqtmlizing connections of low resistance. These 
are connections between points of the winding which should 
be at the same potential, and they usually take the form 
of rings situated at the commutator end of the armature 
core. 

39. E.M.F. Equation of Dynamos. — The average elec- 
tromotive force between brushes, induced in the conductors 
of an armature revolving in a multipolar magnetic field, is 

E &v = 2 <& m Sf io" 8 volts, §33 

where <$> m is the flux per pole in maxwells which cuts the 
conductors, 5 is the number of conductors in series between 
brushes, and /is the number of magnetic cycles through 

which the armature core passes in one second ; or/ = / . 

60 

This equation is perfectly general, and applicable to any 

type of direct-current dynamo with any style of armature 

winding. To ensure its proper application, a consideration 

of the significance of the term 5 for various styles of 

winding is necessary. 

In general, 5 is the total number of conductors on the 

armature divided by the number of current paths through 

the armature between brushes. Reference to §§36 and 



ARMATURES. 



67 



37 shows that the number of paths between brushes for 
various styles of winding are as follows : 



TYPE OF WINDING 


WAVE 


LAP 


Simplex . 
Duplex . 
Triplex . 
Quadruplex 


2 

4 
6 
8 


2P 

AP 
6p 
8p 



As a numerical example, compute the no-load voltage of 
an 8-pole generator having a simplex lap-wound armature 
with a total of 1920 conductors, when revolving at 300 rev. 
per min. The effective magnetic flux per pole is 7 mega- 
maxwells. 

There are 4 pairs of poles, whence the number of con- 
ductors per circuit is 



S = 



1920 



240. 



2X4 

The number of magnetic cycles passed through per second is 

300 



/ 



60 



= 20. 



Therefore the terminal E.M.F. of the generator (neglect- 
ing the resistance drop in windings) is 
7000000 



£av=2 



10 s 



240 . 20 = 672 volts. 



40. Core Construction. — To reduce to a minimum the 
otherwise excessive eddy current loss (§ 29) in armature 
cores, these are constructed of thin discs of soft wrought 
iron or mild steel, which are more or less thoroughly insu- 
lated from each other by the natural oxide or by a coating 
of varnish on the discs. Sometimes, for special machines, 
shellac coatings on the discs, or thin paper sheets between 



68 DYNAMO ELECTRIC MACHINERY. 

them, are applied. Laminating the core in this way does 
not completely prevent the flow of eddy currents, for small 
E.M.F.'s, will still be induced in each lamina which produces 
them. The eddy current loss, being proportional to the 
square of the thickness of laminations, would be lowered by 
using very thin discs. A limit to the reduction of thickness 
is the increased loss due to the higher flux density neces- 
sary, owing to waste of space which is taken up by the in- 
sulation between laminations. 




Fig. 33- 

For the smaller machines, having armatures less than 16 
inches in diameter, the discs are punched in one piece, and 
usually take the form shown in Fig. 33. Sometimes aper- 
tures are provided in the laminations about the axis, and 



ARMATURES. 



69 



these constitute air passages through the core, thus im- 
proving ventilation. These discs are mounted directly 
on the shaft and are 
keyed to prevent turn- 
ing. They are held 
in position by flanges 
of cast steel or cast 
iron, which are pressed 
together by nuts on 
the shaft, as shown in Flg * 34 ' 

Fig. 34, or by bolts passing through, but insulated from, 
the laminations. 

For the larger armatures, the discs consist of a number 
of segments which are assembled on a mechanical support 





Fig. 35. 

called a spider, being attached thereto by inwardly-project- 
ing lugs on the segments. In Fig. 35 is shown in part the 
spider of a 300-K.W. generator with a segment dovetailed 



70 



DYNAMO ELECTRIC MACHINERY. 



to one of the arms. The joints are staggered in the lami- 
nations of successive layers. Fig. 36 illustrates a section 
of the spider of the same machine with the end flanges hold- 
ing the laminations in place. The flanges are shaped so as 
to form a support for the armature winding. The spider, 
which has an extension for supporting the commutator, is 
pressed onto the shaft and keyed. 




Fig. 36. 

To obtain sufficient ventilation in the armature and 
thereby lessen the temperature rise incident to operation, 
it is usual to provide radial ventilating ducts in the core. 
The rotation of the armature causes air to pass in through 
the axial apertures, or spider openings, and out through 
the ventilating ducts as indicated by the arrows in Fig. 37. 
The ducts are formed by separating the laminations at 
intervals by the interposition of blocks of non-magnetic 



ARMATURES. 



71 



material called spacing pieces, A type of spacing piece is 
shown in Fig. 38, which consists of brass strips set edge- 
wise into slots punched in stout core discs and riveted. 




Fig. 37. 



This type is commendable because it gives support to the 
armature teeth. 

Good practice requires the provision of one ventilating 
duct for every 2 to 4 inches of axial core length, the width 




Fig. 38. 



of the ducts varying from three-eighths to five -eighths of 
an inch. 

Fig. 39 illustrates a Westinghouse armature core partly 
assembled on the spider. One of the segments of a disc 



72 



DYNAMO ELECTRIC MACHINERY. 



and a spacing piece are shown leaning against that 
portion of the spider upon which the commutator is to 
be mounted. 




Fig. 39. 



Two types of armature slots are in general use, the 
open slot and the partly closed slot. The former has the 
advantage that the armature conductors may be formed 
into coils, insulated, and readily inserted into the slots, 



I 



— J m — w 



*l 






1 1 1 1 i 



T 7 ? 



Fig. 40. 



whereas the latter type simplifies the matter of securing 
the windings into place. The open slot is more often 
employed on direct-current machines. Fig. 40 shows some 
of the styles of open and partially closed slots ; the re- 



ARMATURES. 73 

cesses at the top of some teeth being provided for the 
insertion of fiber or wooden wedges, which serve to retain 
the conductors in the slots. 

In many machines the windings are held in place by 
binding wires wound around the periphery of the armature. 
Grooves are provided therefor by having some of the discs 
of slightly smaller diameter. The wire used for this pur- 
pose is generally of hard-drawn brass or phosphor bronze, 
and, on railway motors, of steel. It is wound over insulating 
strips, forming a band of several turns, these being often 
soldered together. 

41. Armature Coils. — The armature coils are of copper, 
in the form of either wire or strips, the former being 
usually employed on the smaller machines. For machines 
of large output, it is not advisable to use heavy bars as 
conductors, because of the eddy currents set up in them 
when one side of a coil is momentarily in a stronger field 
than the other. Thus, a number of smaller conductors, 
insulated from each other and connected in parallel at 
the commutator, avoid this condition. 

In multipolar armatures the windings consist of a num- 
ber of similar and interchangable formed coils, which are 
wound on separate collapsible forming blocks. The several 
conductors that constitute one coil are insulated individually 
and are fastened together and wrapped with a few layers 
of insulating tape. For this purpose cotton or linen tape, 
varnished cloth or paper, or micanite are generally used. 
The advantage of formed coils is their superior insulation 
and the facility with which damaged or burned-out coils 
can be removed without disturbing the other coils. Fig. 
41 shows some Western Electric Company formed coils, 
those on the right being for lap-wound and those on the 



74 



DYNAMO ELECTRIC MACHINERY. 



left for wave-wound armatures. In Fig. 42 is depicted an 
armature core ready for winding, with some formed coils 
in different stages of completion. 

One-turn armature coils for large dynamos, having con- 
ductors of large sectional area, which do not permit of 
bending, are composed of two halves individually insulated 
and placed in their respective slots. A copper bridge is 
then bent over the bare ends at the rear, and soldered, 
thus completing the electrical continuity of the coil. 




Fig. 41. 



Before placing the armature coils in the slots, the latter 
are generally lined with insulating material, such as mica- 
nite, fiber, and a paper pulp known as presspahn. The 
thickness of slot insulation depends upon the terminal volt- 
age of the machine, and should be capable of withstanding 
several times this voltage without puncturing. On the 
other hand, the thickness should not be so great as to 
materially lower the space factor of the slot, i.e. the ratio 
of the copper cross-section to the slot area. This factor 
depends upon a number of conditions, but common values 



ARMATURES. 



75 



for machines up to 75 K.W. at voltages from 100 to 600 
may be interpolated from the curves of Fig. 43 given by 
Hobart. 




A partly-wound armature for a 200-K.W., 500-volt gen- 
erator manufactured by the Allis-Chalmers Company is 
shown in Fig. 44. After all the coils are in place, the 



7 6 



DYNAMO ELECTRIC MACHINERY. 



exposed portions of the windings, or the end-connec- 
tions, must be firmly bound to withstand the centrifugal 
force. 

42. Commutators. — The segments or bars of a commu- 
tator are always of drop-forged or hard-drawn copper. 
They must be properly tapered so that when all the seg- 
ments are put together the whole will form a cylindrical 
structure. The insulation between segments is always of 
mica. Of the various grades of mica employed for insulat- 
ing purposes, the amber-colored mica, which must be free 



.-5 
.4 
























~T0* 


5Ct 








/ 


S**^ 




" fiOO 










.3 
.2 
.1 


volt 








/ 

















































40 60 

KILOWATTS 



80 



Fig. 43- 

from iron, is to be preferred. Besides being a good insu- 
lator, amber mica has the additional advantage of wearing 
at the same rate as copper; thus after long use it leaves 
neither elevations nor depressions on the commutator sur- 
face. Not only must the individual segments be well 
insulated from each other, but especially good insulation 
must be provided between the segments and the spider 
upon which they are mounted and the clamping rings 
which hold them in position, because the potential differ- 
ences at these places are the same as the terminal voltage 



ARMATURES. 



77 



of the machine. The usual thicknesses of mica required 
for commutator insulation, in inches, are : 



VOLTAGE OF MACHINE 


300 VOLTS OR LESS 


300-IOOO VOLTS 


Between adjacent Segments . . . 
Between Segments and Spider . . 


0.02 to 0.04 
0.06 to O.I2 


0.04 to 0.06 
0.10 to 0.15 




*lg. 44- 



When the commutator segments and mica strips are 
assembled in place, a pair of temporary steel rings is placed 
around them, the inner one being split into a number of 
sections, as shown in Fig. 45. The screws are tightened 
so that the component parts of the commutator are firmly 
pressed together. A groove is then turned therein, and 
clamping rings of corresponding shape are fitted. These 



78 



DYNAMO ELECTRIC MACHINERY. 



are then bolted on, and the steel rings removed, leaving 
a completed commutator, such as shown in section in 
Fig. 46. Considerable reliance is placed on the clamping 
rings, for these must prevent the possible dislocation of the 
segments due to expansion and contraction which accom- 
pany temperature change, or due to centrifugal force. 




Fig. 45- 



Fig. 46. 



To secure successful operation a commutator must be 
designed with a sufficient number of bars, so that the 
difference of potential between two adjacent bars shall 
not exceed 10 volts. This would mean that a 100-volt 
bipolar machine should have at least 20 bars. The poten- 
tial between the brushes or around half the commutator 
is 100 volts, hence half the commutator must have ten 
bars. 



ARMATURES. 79 

The number of commutator segments to be used depends 
upon the style of winding and the voltage of the machine. 
According to Arnold, this number should never be less 
than 0.037 times the product of the total number of arma- 
ture conductors and the square root of the current per 
armature circuit. The width of a commutator segment 
for good mechanical construction should not be less than 
T \ inch at the periphery. 

Commutators for turbo-generators or other high-speed 
dynamos usually have small diameters, so that the linear 
speed is not excessive ; nevertheless speeds as high as 
8000 ft. per min. are encountered in practice, which is two 
or three times as great as that of the usual low-speed com- 
mutators. The ordinary methods of commutator construc- 
tion for these high speeds are inadequate because of the 
great centrifugal force tending to pull the commutator 
apart. To prevent this action, stout steel rings, well in- 
sulated from the segments, are shrunk on the outside of 
the commutator at several places. High-speed commu- 
tators have a great axial length in order to secure a large 
radiating surface. 

Commutators should be designed with sufficient exposed 
area so as to radiate the heat which is communicated to 
them without too high a temperature elevation. The total 
commutator loss consists of two principal components, the 
loss due to resistance, and that due to friction. 

The transition resistance between the brushes and com- 
mutator causes a drop in voltage at each point of contact. 
This drop depends upon the quality of the brush, and is 
practically independent of the linear speed of the commu- 
tator, the current density at brush contacts, and brush 
pressure. The drop for different grades of brushes varies 



equal to (- 



-I times the product of the following 



80 DYNAMO ELECTRIC MACHINERY. 

between 0.6 and 1.4 volts for the contacts of one polarity. 
Double these values times the current output of the ma- 
chine gives the loss represented by the transition resistances. 

The pressure of the brushes on the commutator should 
be low, thus resulting in a small friction loss. Light pres- 
sure does not materially increase the voltage drop at the 
brushes. The magnitude of the friction loss in watts is 
' 2X t:X 746 \ 
33000 

quantities : the radius of the commutator in feet, the speed 
in revolutions per minute, the coefficient of friction between 
the brushes and the commutator (0.3 for carbon brushes 
and 0.25 for copper brushes), and the sum of the pressures 
of all the brushes upon the commutator. This latter should 
amount to 1.25 lbs. per square inch of rubbing surface. 
Carbon brushes permit a current density of 30 to 70 am- 
peres per square inch of rubbing surface, and copper brushes 
about eight times as much. 

There is an additional loss at the commutator due to the 
sparking at the brushes and to the currents in the short- 
circuited segments. These losses cannot be calculated 
closely, but may be estimated as equal to about six per 
cent of the regular commutator losses. 

Knowing the total losses in the commutator the tem- 
perature rise may be estimated by means of empirical ex- 
pressions, § 68. 

The connections from the armature windings to the 
commutator segments are made by means of metal strips 
called risers, which are firmly clamped and soldered to the 
rear ends of the segments. In Fig. 47 is shown a completed 
Western Electric Company commutator with the risers 
attached. 



ARMATURES. 



81 



After a commutator has been in use for a time, it becomes 
grooved and pitted, a condition which causes further 
sparking and wear, and the commutator must be turned 
down again to a true surface. The design of a com- 
mutator should allow sufficient material for repeated opera- 
tions of this kind. 




Fig. 47- 

43. Brushes and Brush Holders. — Brushes are gener- 
ally made of hard blocks of graphitic carbon. These 
brushes wear well mechanically and give the commutator 
a smooth surface, and further, the greater resistance of a 
carbon brush results in less sparking when it bridges two 
commutator bars than would the lower resistance of a 
copper brush. Carbon brushes are generally set at an 
angle, though some makers set them radially, especially in 
motors which must be reversed in direction, as in the case 
of railway and elevator motors. 



82 



DYNAMO ELECTRIC MACHINERY. 



On low-potential machines brushes of copper gauze are 
sometimes used, because there is less tendency to spark 
on low voltages, and because the resistance of carbon 
brushes would be too great a portion of the resistance of 
the entire circuit. Such brushes are also used on some 
turbo-generators. 

The number of brush sets necessary depends upon the 
style of winding employed on the armature, § 36 ; but 
there may be, and usually are, several brushes per set. 
That is, instead of broad brushes, a number of smaller 
ones are used on all machines except those of little out- 
put. This scheme enables the removal of brushes one 
at a time for trimming purposes while the machine is in 
operation. 

Individual brushes are supported in brush holders, as in 
Fig. 48, which shows a box-guide type of Westinghouse 

manufacture. Brush 
holders should provide 
adjustment as to posi- 
tion and tension of the 
brushes, and allow the 
latter to follow any ir- 
regularity in the com- 
mutator surface. The 
brush-holder springs 
should be arranged so 
that the brush pressure 
is maintained constant 
during the life of the brush. The spring should not 
form a part of the electric circuit ; flexible copper con- 
ductors generally complete the connection from the brush 
to the stationary part of the holder. A General Elec- 




Fig. 48. 



ARMATURES. 83 

trie Company brush-holder arm is shown complete in 
Fig. 49. 

The brush-holder arms are carried on rings, called 
rockers, which are mounted concentric with the commuta- 
tor, either on a sleeve at the front bearing or on the field 




Fig. 49. 

magnet frame. The rocker is capable of being moved 
around the commutator by means of a tangential screw 
and hand wheel. After the proper brush position has been 
found the rocker is securely clamped. All the brushes, both 
positive and negative, are usually mounted on the same 
rocker, so that their adjustment is simultaneously effected. 
Fig. 50 shows such a rocker with the brush gear. 

44. Shafts and Bearings. — As shafts for armatures are 
often subjected to sudden large variations in load, it is 
usual to construct them somewhat larger than those of 
other machines of similar size. The diameter of the shaft 
depends upon the output and speed of the armature, and 
to obtain practical values the following empirical formula 
may be used : 

D = ..W. output 



*/ K.V 
V rev. 



per mm. 



84 



DYNAMO ELECTRIC MACHINERY. 




Fig. 50. 




Fig. 51. 



ARMATURES. 85 

where D s is the shaft diameter in inches, and k is a con- 
stant having the following values for different-sized machines 
using mild-steel shafts : 

50 K.W. or less, k = 6.5 

50 K.W. to 500 K.W., k = 8.4 

500 K.W. and over, k = 10.2 

This shaft diameter refers to that part under the core and 
commutator, the portions within the bearings being some- 
what less. 

Dynamo bearings should have ample bearing surface and 
be rigidly constructed. They are always made in two sec- 
tions, thus permitting the removal of. the armature. It is 
necessary that the bearings be exactly in line, and frequently 
a form of self-alignment bearing is used, as in Fig. 5 1. The 
shaft revolves in a cylindrical brass bearing having an outer 
spherical enlargement at the center which rests upon a cor- 
responding bed of Babbitt metal. 

Lubrication may be secured by the use of ordinary oil 
cups, but generally by the employment of self -oiling devices. 
One of these, Fig. 51, consists of two brass rings playing 
in semi-circumferential slots in the bearing, which permit 
the rings to hang loosely on the shaft. The bearing pedes- 
tals are hollow under the rings and serve as oil receptacles. 
As the shaft revolves, the rings also revolve at such a rate 
as to carry a steady stream of oil up into the slots, thereby 
lubricating the bearing. 

Recently ball bearings have been applied to bearings 
of small dynamos, say up to 50 K.W., and have given 
complete satisfaction. One type of ball-bearing is shown 
in Fig. 52, where is the outer ring and i the inner ring 



86 



DYNAMO ELECTRIC MACHINERY. 




Fig. 52. 



with the hardened polished steel balls, 
by between them. A removable piece, /, 
somewhat longer than the diameter of a 
ball, permits of the insertion of the 
balls, and may be held in place by the 
screw s. The rings are of hardened steel 
and have polished running surfaces. The 
inner ring is rigidly fastened to the 
shaft. 



PROBLEMS. 



i. The instantaneous E.M.F. induced in a conductor of a 
revolving loop at the moment it cuts a certain magnetic flux at 
an angle of 6o° is 2.5 volts. What electromotive force is in- 
duced in this conductor at the instant the angle between the 
paths of flux and conductor is 70 degrees ? 

2. The maximum E.M.F. induced in each of six similar 
loops placed 30 electrical degrees apart, revolving together 
about a common axis in a uniform magnetic field is 25 volts. 
Calculate the maximum and minimum values of the total vol- 
tage resulting from the connection of all the loops in series. 
Determine the percentage fluctuation. 

3. How many magnetic cycles does the armature core of a 
16-pole dynamo pass through per second, if the armature makes 
375 revolutions per minute? 

4. How many inductors may there be on a 12-pole simplex 
wave winding in which the rear winding pitch is 13 and the 
front winding pitch is 17 ? 

5. There are 122 inductors on an eight-circuit short-chord 
simplex winding which is imbedded in 61 armature slots. 
Determine the maximum value of the mean winding pitch, the 
even-numbered conductors being located in the bottoms of the 
slots. 



PROBLEMS. 87 

6. A six-pole multiplex wave winding has 60 inductors and 
a mean winding pitch of 9. What is the degree of multiplicity 
and of re-entrancy of the winding ? 

7. The armature of a 12 -pole generator makes 250 revolu- 
tions per minute. It is simplex lap-wound, and has 180 slots 
with 4 conductors per slot. What is the average value of the 
induced E.M.F. at no load if the magnetic flux passing through 
the armature is 10 megamax wells per pole? 

8. A triplex wave-wound armature with 294 inductors, each 
of four conductors, is substituted for the armature of Prob. 7, 
all other conditions remaining the same. What is the no-load 
terminal voltage ? State the degrees of re-entrancy obtainable 
in this armature winding. 

9. Determine the total commutator losses of a 250 rev. per 
min. dynamo when delivering a current of 300 amperes. Two 
positive and two negative sets of carbon brushes are employed, 
the grade of the brushes being such as to result in a drop of 
1.1 volts at the contacts of one polarity. A current density of 
50 amperes per sq. in. of brush rubbing surface is to be allowed. 
The diameter of the commutator is 18 inches. 



88 DYNAMO ELECTRIC MACHINERY. 



CHAPTER IV. 

FIELD MAGNETS. 

45. Field-Magnet Frames. — In the foregoing chapter 
was shown the dependence of the electromotive force in- 
duced in a dynamo armature upon the total magnetic flux 
cut by the conductors. This magnetic flux is produced by 
the current in the field coils of the machine. The path of 
the flux is called the magnetic circuit, and may be divided 
into three main portions, namely: the iron of the field mag- 
nets, the armature core, and the air gap between armature 
and field-magnet poles. The first of these, which consti- 
tutes the field-magnet frame, may, in most machines, be 
subdivided into three parts, viz.: (1) the field cores, upon 
which the field coils are situated; (2) the yoke, which con- 
nects the field cores at the outer ends; and (3) the pole 
pieces ox pole shoes, which are the enlarged inner ends of 
the field cores. 

The frames of direct-current generators may be cast in 
one piece either of cast iron or cast steel, but it is usual to 
construct the field cores separately from the yoke. The 
choice of material for the yoke, as also for the field cores, 
is governed by considerations of (a) weight, (b) first cost, 
and (c) economy and satisfactory regulation in operation. 
Cast iron has the advantage over cast steel in cheapness, 
but as it is magnetically inferior, more material is necessary 
to carry the required magnetic flux; further, if the field 



FIELD MAGNETS. 89 

poles are also of cast iron, the expenditure for copper will 
be greater, because more turns will be required and each 
turn would be longer than if the better cast steel were used. 
In machines having different parts of the field frame of 
different materials, wrought iron, which is the best available 
magnetic substance, is often employed in the form of punch- 
ings for the cores and pole pieces. 

For multipolar machines the yoke is generally circular in 
shape, of rectangular or elliptical section, and is divided 
either horizontally or vertically into two parts to facilitate 
the removal of the armature, the two halves normally being 
bolted together. It is mounted upon a cast-iron bed plate, 
to which are also fastened the pedestals which carry the 
armature bearings. The bipolar type of machine is now 
restricted to the high-speed smaller units, say 5 K. W. or 
less, because of the greater amount of necessary material 
occasioned by the longer magnetic circuit. Bipolar field- 
magnet frames are made in a great variety of shapes, some 
of which are shown in Figs. 21, 22, and 23; and are gen- 
erally cast in one or more pieces which are bolted together 
after the field coils are in place. 

Separate cores for multipolar frames are constructed 
of cast steel, laminated wrought iron, or laminated steel. 
Solid cores are usually of circular or of rectangular cross- 
section, and laminated cores are of the latter only. A pole 
piece, built up of soft-steel laminations riveted together 
between stout end plates, is shown in Fig. 53, which is 
representative of Westinghouse practice. Field cores may 
either be fastened to the yoke by bolts passing through 
the frame and screwed into the core or pole pieces, or be 
cast integral with the yoke. The latter method gives es- 
pecially good magnetic joints, but the former allows the 



90 DYNAMO ELECTRIC MACHINERY. 

removal of any one of the cores with its field coil for pur- 
poses of repair. 

The pole tips are generally somewhat larger than the 
cores, as shown in Fig. 53, an arrangement which serves 




Fig. 53 

the double purpose of producing a more uniform flux dis- 
tribution in the air gap, and retaining the field coils in 
position. In this particular type, one corner of each 
punching is cut away and the laminations are stacked with 
the beveled corners alternately to one side and to the other, 
thus producing a pole with saturated pole tips, which is 
advantageous in yielding good commutation. Where solid 
poles of cast iron or cast steel are used with separate pole 
pieces, the latter are preferably made of laminated wrought 
iron, thereby reducing the detrimental effects of eddy cur- 
rents induced in the pole faces by the flux variation occa- 
sioned by the slots in the revolving armature core. 

In the usual designs of direct-current dynamos, the ratio 
of the peripheral length of the pole face to the pole pitch, 



FIELD MAGNETS. 91 

or distance between adjacent pole centers, ranges from 0.60 
to 0.75, and this ratio is known as the polar span; that is, 
the polar span is from 60% to 75% of the pole pitch. 




Fig. 54- 

Fig. 54 shows the complete field-magnet frame of a 
Crocker-Wheeler generator. This frame is of cast iron, 
and the round poles are of cast steel provided with remov- 
able cast-iron shoes, which are clamped in place after the 
field coils have been assembled. 



9 2 



DYNAMO ELECTRIC MACHINERY. 



46. Methods of Field Excitation. — Dynamos are classi- 
fied according to the five methods of exciting the fields of 
the machine. They are : — the Magneto, the Separately 
Excited, the Shunt Wound, the Series Wound, and the 
Compound Wound. The last three methods refer to self- 
exciting machines, that is, generators which supply their 
own field current. 

The magneto generator, Fig. 55, is one in which the 
field is a permanent steel magnet, generally of horseshoe 
form. Because of the low flux densities in this type of 





MAGNETO DYNAMO 
Fig* 55- 



SEPARATELY EXCITED DYNAMO 

Fig. 56. 



machine necessitating more iron, its use is limited to small 
dynamos, such as those used in gasolene engine ignition 
work, or for telephone signaling. 

The separately excited dynamo, Fig. 56, has, as its name 
implies, its field coils traversed by a current other than 
that produced by the machine. It is produced by an aux- 
iliary generator called an exciter. Alternating-current ma- 
chines are nearly always of this type. 

The shunt -wound machine, Fig. 57, has a large number 
of turns of fine wire wound on its field core, and the ends 
are connected to the terminals of the machine, thus being 



FIELD MAGNETS. 



93 



in shunt with the outside circuit. The ampere-turns requi- 
site for excitation are obtained by passing a small current, 
say from 2 to 8 per cent of the total current output, through 
a large number of turns. 



/ 




\ 






l< 


t 

^ 


V 




1 





/ 




> 




< 

< 
< 
c 


< 

< 
» 

c 
> < 


l< 


d 


V 



SHUNT WOUND 
DYNAMO 



Fig. 57- 



SERIES WOUND 
DYNAMO 

Fig. 58. 



The series-wound generator, Fig. 58, has all the current 
that is produced by the armature passing through a few 
turns of large wire wound around the field cores. The 
exciting coils are then in series with the external circuit. 
The ampere-turns required for excitation are obtained by 
passing a large current through a small number of turns. 
Series generators are practically only used for series arc 
lighting and in the Thury system of direct-current power 
transmission at high voltages. 

The compound-wound machine, Fig. 59, is one in which 
there are both shunt and series coils on the field magnets. 
This method of winding is used for purposes of regulation 
under varying loads, as will be explained later. Compound 
windings are of two classes, the long shunt and the short 



94 



DYNAMO ELECTRIC MACHINERY. 



shunt. In the former, the current used in the shunt wind- 
ings is also passed through the series windings along with 
the main current. In the latter, the current from the 
shunt coils passes directly back to the armature, avoiding 



y^ 







< 
< 
< 

i 


> 


t 




w 



COMPOUND WOUND 
DYNAMO LONG SHUNT 



/^ 




\ 


\ 








m 


V 



Fig. 59- 



COMPOUND WOUND 
DYNAMO SHORT SHUNT 

Fig. 6o. 



the series turns. Figs. 59 and 60 clearly show the con- 
nections of the two types. The short-shunt compound 
winding is generally preferred. 

Shunt-wound and compound-wound generators find their 
principal utilization in constant-potential systems of elec- 
trical distribution for lighting and power. 

47. Magnetic Leakage. — Since air is not an insulator 
of magnetism, but is simply much less permeable than 
iron, it is evident that some of the lines of force generated 
by the field coils will not follow around the desired path 
through pole pieces and armature, but will take a path 
through the air and be of no utility in creating E.M.F. in 
the revolving armature. Fig. 61 roughly represents some 
of the paths such lines may take. 



FIELD MAGNETS. 



95 



If <& t be the total flux set up by the field coils, and <£> a be 
that portion of it which passes through the armature and 
is cut by the conductors, then the coefficient of magnetic 
leakage, or dispersion coefficient, is 



d = 



and is always greater than unity. 




Fig. 61. 



In practice, the value of d ranges from 1.25 to 1.5 in 
bipolar dynamos, depending upon the design. For multi- 
polar machines, the values of the dispersion coefficient 
vary from 1.3 in small dynamos of about 2 K.W. to 1.1 
in large machines of about 500 K.W. output. 

Increasing the field excitation of a generator results in 
more magnetic flux, not all of which is useful in developing 
a greater voltage. Further, the leakage flux increases 
faster than the useful flux ; for as the flux density of the 
magnetic circuit becomes greater its permeability drops, 
whereas the permeability of air is constant and unity at 
all flux densities. The dispersion coefficient is therefore 
dependent upon the load on the machine. 



g6 DYNAMO ELECTRIC MACHINERY. 

The dispersion coefficients of small machines for definite 
excitation may be determined experimentally by the use of 
test coils in connection with a ballistic galvanometer. 

48. Calculation of Exciting Ampere-Turns. — In order 
that a generator armature may produce the desired voltage 
when revolving at a definite speed, a certain amount of 
magnetic flux must be cut by the armature conductors. 
This flux is set up in the magnetic circuit of the machine 
by a current flowing through the field coils. The strength 
of current necessary and the number of turns of wire to be 
provided on the field coils depend upon the length and 
reluctance of the magnetic circuit and the dispersion coef- 
ficient. As the reluctances of the various portions of 
the magnetic circuit are different owing to differences of 
dimensions, flux density, or permeability, it is necessary to 
calculate the magnetomotive force, or ampere-turns, for 
each of the sections; their summation will then determine 
the required exciting ampere-turns, after correcting for 
magnetic leakage. 

In designing the magnetic circuit of a dynamo every 
portion of it should have sufficient cross-section to carry 
the total flux at a reasonable flux density. It is assumed 
in such calculations that all the magnetic leakage occurs 
at the pole faces ; hence the total flux which passes through 
the armature <core and teeth from the air gap is the useful 
flux, 4> a , and that which passes through the center of the 
field cores is the total flux produced, or <Jy = d$> a . This 
assumption simplifies the calculation without affecting the 
degree of accuracy to any great extent. 

In the following table are given the various parts of the 
magnetic circuits of ordinary direct-current multipolar 
machines, with the usual materials of which they are 



FIELD MAGNETS. 



97 



constructed, and the common values of flux density 
therein. 



PORTION OF 




FLUX DENSITIES 




MAGNETIC CIRCUIT 




KILOMAXWELLS PER SO. 


IN. 


Armature Core 


Soft Laminated Iron 


70 to I 10 




Armature Teeth 


Soft Laminated Iron 


100 to 140 




Air Gap 


Air 


40 to 60 




( 


Cast iron 


30 to 50 




Field Cores < 


Cast steel 


70 to 100 




1 


Soft Laminated Iron 


70 to 110 




Field-magnet Yokes } 


Cast iron 
Cast steel 


27 to 45 
70 to 90 





Having decided upon the dimensions and material of 
each portion of the magnetic circuit, the ampere-turns 
required to drive the flux through them is calculated from 
the expression 



nl = 



IO (R<£ 



0.313 



Z(B 



§§ 23-27 



4-2.54 ii 

where for a given portion (R is the reluctance in oersteds, 
/ is the length in inches, (B is the flux density in maxwells 
per square inch, <f> is the total flux in maxwells, and p. is 
the permeability of the material. In multipolar machines 
it is only necessary to consider one complete magnetic 
circuit, that is, for one pair of poles. The resulting ampere- 
turns are those necessary for two field coils. 

As an example determine the ampere-turns per pole 
required to send a flux of 20 megamaxwells through the 
armature of the 350-K.W., 500-volt, 8-pole generator 
shown in part in Figs. 62 and 63, in which the dimensions 
are expressed in inches. The mean path of the flux is 
shown by the closed dotted line. 

Armature Core. To allow for the insulation between 
the armature laminations it is usual, in practice, to con- 



98 



DYNAMO ELECTRIC MACHINERY. 




""A " 










up 








..!.... 






FIELD MAGNETS. 99 

sider that it occupies 10% of the gross length of iron. 
The presence of ventilating ducts further decreases the 
net iron length. The cross-sectional area of the armature 
core is 

A c = (14 - 1.2) X 0.9 (15 - 4 X 0.5) = 150 sq. in. 

In multipolar machines of the usual construction there 
are two paths for the flux per pole through the armature 
core and field yoke ; hence only one-half of the flux per 
pole passes through these parts. The flux density in the 
armature core is 

20,000,000 ,.. ,■ 11 

(Be = = 00,000 maxwells per sq. in. 

150x2 

The permeability of the core at this flux density, as 
determined from Fig. 10, is fi c = 1650. 

The mean length of path traversed by the flux is 
approximately 33 inches. 

Therefore the ampere-turns required to overcome the 
armature core reluctance are 

= 0.313x33x66,600 = 

v 1650 

Armature Teeth. The accurate determination of the 
ampere-turns necessary to send the magnetic flux through 
the armature teeth is very complicated, but the following 
practical method, involving a few assumptions, leads to 

good results. The number of teeth under a pole piece is 

00 
21 -^ ^18.2. Owing to the fringing of the flux at the 

240 

pole tips, not merely the teeth immediately under the pole 
face carry the flux from that pole, but one or more addi- 
tional teeth may assist. Making an allowance of 10 per 



100 DYNAMO ELECTRIC MACHINERY. 

cent for this flux fringing, a value frequently taken, then 
the number of armature teeth receiving flux from one pole 
is 18.2 X 1.1 = 20. 

As the teeth are wider at the periphery than at the bot- 
tom of the slots, their sectional area will be different at 
various distances from the axis of rotation. It is usual to 
employ the sectional area at one-third the tooth length from 
the narrow end. The width of the tooth at this place is 

j 88 -( 2 -^ ci. 2 yi 

_i - Li o.c6 0.57 inch. 

240 ° 

The net cross-sectional area of the flux-carrying teeth per 
pole is 

A t = 20 X 0.57 X 0.9 (15 - 4 v 0.5) 134 sq. in. 

Armature teeth are worked at high flux densities, and 
at these densities the permeability of iron is not very high. 
Consequently the permeance of the copper or air between 
the teeth cannot be neglected, and a correction must be 
made therefor. The terms apparent and corrected flux 
densities are to be distinguished in this connection. Thus, 
the apparent flux density in the teeth is 

20,000,000 

(R ta = _ = 149,000 maxwells per sq. in. 

134 

The corresponding corrected flux density may be obtained 
from the curves of Fig. 64 given by Hobart. The differ- 
ent curves refer to different ratios of tooth-width to slot- 
width. For the dynamo under consideration, in which the 
tooth-width and slot-width are practically equal, the cor- 
rected tooth flux density is 

(B <c = 138,000 maxwells per sq. in. 



FIELD MAGNETS. 



101 



150 



3130 



■120 



110 















tJ tooth-width 

W s = SLOT-WIDTH 






- = 1 


s = - 75 

-=.5 
























V 




















































































y 


# 


<?' 




















A 


# 


/ 




















^ 


'? 

























120 



130 140 150 160 

APPARENT FLUX DENSITY IN 
KILOMAXWELLS PER SQ.IN. 



170 



Fig. 64. 



150 



120 



110 



200 
20 



400 
40 



600 
60 





\ 


































<$>. 


f. 
































Si 


EET IR 


ON 










































J? 





























































































23.3 



21.7 

20.2 



18.6 



1000 1200 3C 

100 120 /* 



Fig. 65. 



102 DYNAMO ELECTRIC MACHINERY. 

The permeability of the iron at this flux density, as 
determined from Fig. 65, is p.t = 30. 

The length of two teeth is l t = 2 X 1.2 = 2.4. 

Hence the ampere-turns required to overcome the re- 
luctance of the armature teeth are 

(nI)t . 0.313x2.4^138,000 _ 345a 

Air Gap. The usual practice in computing the flux 
density in the air gap is to divide the total flux per pole 
entering the armature by the area of the pole face, thus : 

20,000,000 - ., 

<$>„ = = co, 000 maxwells per sq. in. 

a 17x21 D v ^ 

The radial length of the air gap is 0.3 inch. 

Since the permeability of air is unity regardless of flux 
density, the ampere-turns required to overcome the air-gap 
reluctance are 

(nl) a = 0.313 X 0.3 X 2 X 56,000 = 10,500, 

which is the principal component of the total number of 
ampere-turns to be provided on the machine. 

Field Cores. Taking a value of 1 . 1 5 f or the dispersion 
coefficient, the total flux traversing a field core will be 
20,000,000 X 1.1 5 = 23,000,000 maxwells. The sectional 
area of the poles is 

A v = 17 X 14 = 238 sq. in. 
Therefore the flux density in the poles is 

23,000,000 „ . .. 

(B p = -^ ' = 96,000 maxwells per sq. m. 

238 

The permeability of cast steel at this flux density, as 
determined from Fig. 12, is p p = 480. 



FIELD MAGNETS. 103 

The length of two field cores, including pole pieces, is 
35.4 inches. 

Therefore the ampere-turns required to overcome the 
reluctance of the field poles are 

( W J)^ a3I3X3 y o X96 ' 600 = 2 2 3 o. 

Field Yoke. The cross-sectional area of the yoke is 
12x25 = 300 sq. in., and the flux traversing it is 1 X 23,000,- 
000= 11,500,000 maxwells. Therefore the flux density is 

m 11,500,000 „ 

($> y = — = 38,300 maxwells per sq. in. 

The permeability of the cast-iron yoke at this flux den- 
sity, as determined from Fig. 11, is ^=253. 

The mean length of path traversed by the flux is approxi- 
mately 54 inches. 

Hence the ampere-turns required to overcome the yoke 
reluctance are 

{nI) 0.313 x 54x38,300 6a 

253 

Summary. The ampere-turns per pair of poles neces- 
sary for overcoming the reluctance of the entire magnetic 
circuit and the various parts of it are as follows : — 



Armature Core, 


420 


Armature Teeth, 


3>450 


Air Gap, 


10,500 


Field Cores, 


2,230 


Field Yoke, 


2,560 


Total, 


19,160 ampere-turns. 



104 DYNAMO ELECTRIC MACHINERY. 

Therefore 9580 ampere-turns must be provided on each 
pole in order to set up the required flux. This is true only 
when the flux distribution through each section is uniform, 
which condition exists when no current flows through the 
armature, that is, at no load. Additional ampere-turns must 
be provided when the generator delivers energy so as to 
neutralize the effects of demagnetization and distortion, 
which will be discussed in Chapter V. 

49. Field Coils. - In a shunt-wound machine, the am- 
pere-turns necessary for excitation are obtained by a rela- 
tively small current flowing through many turns of wire, 
whereas in a series-wound machine the required ampere- 
turns are obtained by sending the entire current, or a definite 
part of it, through but a few turns of wire. In a compound- 
wound machine, the shunt winding supplies the ampere- 
turns required to produce the definite magnetic flux through 
the armature at no load, and the series winding supplies the 
additional ampere-turns necessary for full-load operation. 

Knowing the necessary ampere-turns per field pole at 
no load, the size or cross-section of the wire to be used 
for the shunt-field winding may be calculated as follows : 

Let I sh = current in the shunt-field winding in amperes, 
E = terminal voltage of machine at no load, 
and E rh = voltage allowance in regulating rheostat ; 

then the resistance of each shunt-field coil in ohms is 

E - Erh ( x 

where/ is the number of pairs of poles. 

The temperature rise of the field coils under full-load 
operation should not exceed 50 C. above the usual engine- 



FIELD MAGNETS. 105 

room temperature of 25°C. At a temperature of 75° C. 
the resistance between opposite faces of an inch cube of 

copper is '^ ( 1 + 0.0042 x 75) =0.825 microhms (§ 4.) 

2 -54 
Representing the number of turns on one shunt coil by 
n sh , the mean length of a turn in inches by L sh , and the 
section of the conductor in square inches by A s h, then the 
resistance of each shunt-field coil in ohms is 

0.825 f^Z,^ 
Rsh = — ^7fi~A W 

10 Ash 

Equating (1) and (2) it follows that the conductor cross- 
section of the shunt winding is 

Q.S2SL sh (nI) sh 2p 
Ash== (E-E rh )io« Sq ' m - (3) 

If circular conductors are to be employed the proper size 
may be determined by reference to a wire table. 

As an example on the foregoing, calculate the section 
of the shunt-field conductor necessary to provide 9580 
ampere-turns on each field pole of the 350-K.W., 8-pole, 
500-volt generator of § 48. 

Assume 15% of the generator voltage to be taken 
up by the adjusting rheostat, and that the depth of the 
field winding is 3 inches. The mean length of a turn is 
then 74 inches. Therefore the conductor area of the 
shunt winding is 

. 0.825 X 74 X 9580 X 8 
A sh = -, — — ~— R = 0.011 sq. in. 

(5oo-75)io 6 ^ 

The space occupied by the conductors depends upon the 
number of poles, voltage, and speed of the machine, and upon 



I06 DYNAMO ELECTRIC MACHINERY 




Fig. 66. 




Fig. 67. 



FIELD MAGNETS. 107 

the shape of the conductor section. As a rule, from 40% 
to 70% of the total cross-sectional area of a field coil is 
occupied by copper, the remainder being taken up by 
insulation. The ratio of the total copper section to the 
gross section of the coil is called the space factor of the 
coil. 

Having determined the space factor of a proposed shunt- 
field winding, the length of the coils may be computed, 
and the available space for the series winding, if any, may 
be obtained. Then the calculation of the number of turns 
and the size of conductor on the series winding follows, 
the method of procedure being similar to that for the 
shunt winding. 

Shunt coils are usually made of cotton-insulated round 
or rectangular conductors, wound on metal frames or on 
removable molds and held in shape by paper and rope 
bands, the exterior of the coils being coated with moisture- 
proof insulating varnish. The coils of series machines 
and the series coils of compound-wound dynamos are very 
often of forged copper conductors insulated with tape, 
the individual turns being separated by spacing pieces. 
Typical Westinghouse series and shunt-field coils are 
shown in Fig. 66. The current density in shunt coils of 
the usual construction is about 1000 amperes per square 
inch, and in series coils it may be 20 % greater because 
of the superior heat-radiating facilities. 

A completed and assembled field winding of a 250- 
K.W., 250-volt, compound- wound generator is depicted 
in Fig. 67, in which are shown the connections of the 
series field coils at the end of the machine away from 
the commutator. 



108 DYNAMO ELECTRIC MACHINERY. 

PROBLEMS. 

i. The resistance of the field winding of a 15-K.W., 220- 
volt shunt-wound generator is 60 ohms. What percentage of 
the full-load power output is the power consumed in field 
excitation ? 

2. A series-wound generator supplies current to five arc 
lamps connected in series, each taking 9.6 amperes at 47 volts. 
The resistance of the field winding is 1.2 ohms, and the drop in 
the external circuit is 15 volts. Calculate the percentage of the 
total power expended in exciting the field magnets. 
. 3. The flux density in the field-magnet cores, which are 
6 inches in diameter, of a bipolar dynamo is 80 kilomaxwells per 
sq. in., and the dispersion coefficient of the machine as deter- 
mined experimentally is 1.35. Calculate the magnitude of the 
flux passing through the armature. 

4. Calculate the number of no-load ampere-turns to be pro- 
vided on each field pole so that a flux of 6 megamaxwells per 
pole may be sent through the armature of a 100-K.W., 6-pole, 
no-volt, compound-wound dynamo having the following con- 
stants and dimensions : 

Armature outside diameter = 32 in. 

Armature internal diameter = 20 in. 

Armature gross length = 9 in. 

Two ventilating ducts, each 0.4 in. wide. 

125 armature slots, each 1 in. X 0.4 in. 

Armature core of laminated iron. 

Radial length of air gap = 0.2 in. 

Dispersion coefficient = 1.20. 

Diameter of field cores = 9.5 in. 

Polar span = 70%. 

Field structure of cast steel. 

Internal diameter of yoke = 54 in. 

Outside diameter of yoke == 62 in. 

Yoke width = 10 in. 



PROBLEMS. 109 

5. Allowing 20% of the voltage of the generator of Prob. 4 
to be lose in the adjusting rheostat, determine the size of cir- 
cular copper wire to be used in the shunt winding. The outside 
diameter of the field coils is 13 inches, and the net length of 
the winding is 9 inches. 

6. Calculate the total resistance and number of turns of the 
shunt winding of the generator of the two foregoing problems, 
the space factor of the field coils being 0.60. 



IIO DYNAMO ELECTRIC MACHINERY. 



CHAPTER V. 

ARMATURE REACTION. COMMUTATION. 

50. Armature Reaction. — ■ When an armature revolves 
in a magnetic field an electromotive force is developed in 
its conductors, and this is available at the brushes of the 
machine. If, therefore, the brushes be connected through 
an external circuit, a current will flow through it, and the 
current strength will depend upon the resistance of the cir- 
cuit. This current, in flowing through the armature wind- 
ing, will exert a magnetizing action on the core independently 
of the field magnets ; thus there are two coexistent mag- 
netic fields with directions at an angle to each other. As 
the resultant field differs in direction from that caused 
only by the current in the field coils, it is frequently said 
that the current in the armature conductors causes a dis- 
tortion of the flux in the magnetic circuit, and this effect 
of the armature current is called the cross-magnetizing 
effect. Because of cross-magnetization, it is necessary, in 
order to secure good commutation, to shift the position of 
the brushes away from the geometrical median line be- 
tween radii to two adjacent poles. A shifting of brushes 
from this neutral plane results in a weakening of the mag- 
netic field, due to the armature current in some of the 
conductors, and this effect is called the demagnetizing 
effect. These two effects of the armature current, i.e., the 
cross-magnetizing and demagnetizing effects, when consid- 
ered together, are called armature reaction or armature 
interference. 



ARMATURE REACTION, 



III 



51. Cross-Magnetizing Effect of Armature Current. — 

Consider a drum armature revolving in a bipolar magnetic 
field, and let the brushes be placed midway between the 
poles, that is, in the neu- 



Q(-~) 




tral plane. When the 
field magnets are excited 
and the armature is run- 
ning on open circuit, 
the magnetic flux which 
passes through the arma- 
ture core may be repre- 
sented by dotted lines in 
Fig. 68, and it is seen 
to be quite uniformly distributed. Upon interrupting the 
exciting circuit, and sending a current from some outside 

source through the gener- 
ator armature, the resulting 
magnetic field may be de- 
picted as in Fig. 69. This 
is called the cross jlitx 
because its axis is across 
that of the main field flux. 
In operation, however, both 
of these fields exist simul- 
taneously, and the resultant 
flux through the armature 
is obtained by combining these two conditions as in Fig. 70. 
The flux distribution through the generator field poles and 
armature is thus non-uniform, and the distortion takes place 
in the direction of rotation, resulting in the crowding of 
lines of force in the trailing pole tips. This distortion of 
the magnetic flux occasions a loss in the operation of a 





\ 


r / 




00 






00 


'"~7 


'&^ 


!%■' 


\ "* ^^ 




!jK s% v 


' f '\3 


)V S \ 


1 ' ;\k 

I 1 V \% 




'iiii^ 


0\\ \ \ 




1 ' 1 ' 

•X ' ' 


» \ * v o 


\1' / 


v --^ 






/--' 


00 


uu 




/ V 







Fig. 69. 



112 



DYNAMO ELECTRIC MACHINERY. 



generator because it increases the reluctance in two ways, 
— (a) by saturating the iron at the pole pieces and thus 




reducing the permeability, and (b) by lengthening the paths, 
both in air and in iron, that the lines of force follow. 




Fig. 7i- 



The composition of the two apparent magnetic fluxes in 
the air-gap of a bipolar dynamo is illustrated in Fig. 71, 
which shows rectified curves of magnetic distribution under 
the pole pieces and in the air-gap of the machine, ordinates 



ARMATURE REACTION. 113 

of the curves representing flux density. Curve a represents 
the symmetrical flux distribution occasioned by the current 
in the field magnets, and curve b shows the flux distribution 
in the air-gap due to the armature current. Adding the 
ordinates of these curves yields the resultant flux dis- 
tribution actually existing in the air-gap of dynamos under 
load when brushes are in neutral plane, as indicated in 
curve c. 

52. Demagnetizing Effect of Armature Current. — The 
effect of the cross flux is to produce with the main flux 
a resultant magnetic field in the air-gap of a dynamo the 
direction of which is inclined to the direction of the field 
flux, as shown by the arrows in Fig. 70. Since the brushes 
should be at those points where they may short-circuit coils 
whose conductors begin to cut magnetic lines of force in a 
reversed direction, the position of the brushes should not be 
midway between the poles (or in the neutral plane), but at 
right angles to the direction of the resultant flux. Therefore 
the plane of the brushes through the armature axis, known 
as the commutating plane, should be shifted away from the 
neutral plane for a generator in the direction of rotation. 
This shifting of the commutating plane causes a further 
distortion of the flux, necessitating a continued shifting 
of the brushes until the commutating plane is displaced 
90 electrical degrees from the final resultant flux. In 
the foregoing, the armature conductors are supposed to 
be connected to commutator segments lying on the same 
radius. 

For generators, the commutating plane is in advance of 
the neutral plane, and for motors, behind the neutral plane. 
The angle between these planes is termed the angle of brush 
lead or brush lag. 



H4 



DYNAMO ELECTRIC MACHINERY. 



In Fig. 72 is shown a generator armature revolving in a 
bipolar field, the brushes being given a forward displacement 
of degrees. The armature conductors may be divided into 
two separate groups by the two vertical lines ab passing 




Fig. 72. 



through the brush contacts, each group being considered as 
forming a number of complete turns. The current flowing 
in the turns which lie outside of these lines sets up the 
cross flux as shown in § 51, and the current flowing through 
the turns included between the lines ab produces a mag- 
netomotive force opposing that of the field-magnet coils, 
and thus exerting a demagnetizing effect on the magnetic 
circuit. The product of the number of turns between aa 
and the current flowing in them is called the demagnetizing 
or back ampere-turns, since it produces a weakening of the 
magnetic field. 

The current flowing in the armature conductors of a 
multipolar dynamo similarly exerts distorting and demag- 
netizing effects upon the magnetic field, and the armature 
turns may also be divided into two belts, namely cross turns 
and back turns. 



ARMATURE REACTION. 115 

53. Compensation for Armature Reaction. — To neutral- 
ize the effects of cross-magnetization and demagnetization 
produced by the current flowing in dynamo armatures, it is 
necessary to provide additional magnetomotive force by in- 
creasing the ampere-turns of the field-magnet coils. Com- 
pensation for demagnetization is easily calculated, since the 
number of back turns times the current flowing through them 
at any load, multiplied by the dispersion coefficient at that 
load, gives the additional number of ampere-turns necessary 
at that load for compensation. The additional field ampere- 
turns per pole necessary to overcome the demagnetizing 
effect of the armature current at full load may be repre- 
sented by 

(w/) ^ = 78r5-2 '^* 

where = brush lead in electrical degrees (angular distance 
between centers of like-named poles = 360 
electrical degrees), 
5 = number of armature conductors in series be- 
tween two brushes (§ 39), 
/= full-load current of dynamo, leaving or enter- 
ing at the brushes, 
and d = dispersion coefficient. 

The additional field ampere-turns necessary to neutralize 
the distortional effect of the armature current are difficult to 
determine, but the following method of calculation, based 
upon experimental data, yields a good estimation. The 
ampere-turns of the armature producing the cross-magneti- 
zation, being due to the current in the cross turns only, are 

D' = ( 1 - 2 — ) — / per pole. 
V 180/4^ 



n6 



DYNAMO ELECTRIC MACHINERY. 



The ratios of this quantity D' to the number of ampere- 
turns per pole on the field magnets required for compensa- 
tion of distortion, (nl) cm , are the ordinates of Fig. 73; and 
the abscissae represent the no-load ampere-turns per pole 
on the field-magnet coils. This ratio, for values of D' be- 
tween 1000 (lower curve) and 10,000 (upper curve), lies 
between the two curves, its magnitude being determined by 
interpolation. Knowing the no-load field ampere-turns, and 
having calculated the value of D' from the above equation 
at full-load current, the number of ampere-turns per field 
spool, (nl) cm) necessary to compensate for distortion due to 



8 






















\ 














6 




\ 














D' 4 




v 


\o- 












(nl) cm 




\ 


X 




















ijoo^ 















































NO-LOAD FIELD 
KILO-AMPERE-TURNS 



16 



Fig. 73- 



cross-magnetization at this load, may therefore be obtained 
from Fig. 73. 

As a numerical example of the foregoing, calculate the 
field ampere-turns per pole necessary to compensate for 
armature reaction at full load in the 350-K.W., 500-volt 
generator of §48, when the angle of brush lead is 5 degrees. 



ARMATURE REACTION. 117 

The armature has an eight-circuit winding embedded in 
240 slots, there being 6 conductors per slot. The number 
of armature conductors in series between positive and nega- 
tive brushes is 240 X 6 

o = = IoO. 

2X4 

The full-load current of the dynamo is 

350 X 1000 

I = — = 700 amperes. 

500 

Therefore the field ampere-turns necessary to neutralize de- 
magnetization at full load are 

/ T x 2X5 180 

(nl) dm = — 2 X -—^ X 700 X 1.1 5 = 500. 

The ampere-turns of the armature producing cross-magneti- 
zation are 

^/ / 2 x 5\ 180 

D' = ( 1 ^ X X 700 = 7430. 

V 180 / 4 X 4 

The field ampere-turns per pole at no load = 9580 (§ 48). 

With this value as abscissa, the ordinate corresponding to 

77 = 7430 would be 2.8. Hence 

r^- = 2.8, 

from which the ampere-turns per field pole required for 
overcoming effect of distortion at full load are (nl) cm = 

743° r 

L^— = 2650. 
2.8 D 

The total ampere-turns, then, to be provided on each field 
pole are 9580 + 500 + 2650= 12,730. When a dynamo de- 
livers current there is a resistance drop in the machine 
itself, so that the actual induced voltage must be somewhat 
larger than the rated terminal voltage. The additional 



n8 



DYNAMO ELECTRIC MACHINERY. 



ampere-turns required for producing the greater magnetic 
flux necessary to obtain the higher internal voltage have 
not been considered in the foregoing. 

54. Devices for Reducing Armature Reaction. — The 
distortion of the magnetic field may be diminished by 
lengthening the air-gap and working the armature teeth at 
high flux densities, thereby increasing the reluctance of the 
path of the cross flux. This, however, also increases the 
reluctance of the main flux path and necessitates the pro- 
vision of a greater number of ampere-turns on the field- 
magnet coils. 





Fig. 74- 

Magnetic field distortion may be reduced by slotting the 
field poles longitudinally. This considerably increases the 
reluctance of the path of the cross flux by introducing in it 
an air-gap, whereas the reluctance of the main flux path is 
very slightly altered. Fig. 74 shows a Lundell split -pole 



ARMATURE REACTION. 1 19 

type of generator in section and illustrates the construction 
of the pole piece. The magnetic flux which enters the 
pole piece divides between the two paths a and b. Owing, 
however, to the greater span covered by the shoe belonging 
to the part marked b, the magnetic reluctance of that part 
is much smaller than that of the part marked a. As a result, 
the flux does not divide itself equally between the two paths. 
The part of the pole piece marked b under increasing exci- 
tation becomes saturated before the part marked a. At 
normal excitation, the flux density at b is above 16,000 lines 
per square centimeter, while the flux density in a is but 
about 10,000 lines per square centimeter. In other words, 
b is well saturated, while the magnetization of a is still below 
the knee of the magnetization curve. This saturation of 
half of the pole piece is effective in preventing a skewing 
of the field by the cross turns as the load on the machine 
increases. This is shown in the flux distribution curve of 
Fig. 75, wherein the arrows show the direction of rotation 



Fig. 75- 

of the armature. The dotted line represents the distribu- 
tion at no load, and the heavy line the distribution at full 
load. This small distorting effect of the cross turns permits 
the employment of a small air-gap without causing serious 
sparking. 



120 DYNAMO ELECTRIC MACHINERY. 

Ryan compensates for the magnetizing effects of the 
armature winding by surrounding the armature with a sta- 
tionary winding, which passes through perforations in the 
pole faces. These stationary windings carry the whole 
current of the machine, being connected in series with the 
external circuit. The current in these windings produces 
a magnetomotive force equal and opposite to that due to 
the armature current. The number of ampere-turns on the 
compensating winding is about one and one-quarter times 
the armature cross ampere-turns. This arrangement is not 
much used in direct-current practice because of construc- 
tive difficulties and increased cost. 

Distortion of magnetic field is further decreased by prop- 
erly shaping the pole pieces. The distribution of flux 
should be such that an armature coil at first enters a weak 
field and then gradually comes to the strongest part. If 
the lines of force are allowed to crowd into the trailing-pole 
tips, this gradual transition is impossible. If the horns are 
farther from the armature surface than the body of the pole 
face, then the air-gap, and consequently the reluctance at 
the horns, is increased, and the lines of force are dis- 
tributed more symmetrically. The poles may have cham- 
fered corners or be non-concentric with the armature, 
the radius of the latter being less than that of the pole 
faces. 

The use of auxiliary poles between the main field poles 
of dynamos is also effective in reducing armature reaction. 
The coils on the auxiliary poles are connected in series 
with the armature, and the entire current, or a definite part 
of it, traverses the auxiliary winding. This arrangement 
yields perfect commutation at all loads for various speeds 
with a definite setting of the brushes. The field structure 



COMMUTATION. 



121 



of a dynamo with auxiliary poles manufactured by the 
Electro-Dynamic Company is shown in Fig. 76. 




Fig. 76. 

55. Commutation. — The electromotive force induced in 
the armature conductors of practically all direct-current 
generators is alternating, and in order to obtain a unidirec- 
tional E.M.F. at the terminals of the machine it is necessary 
to reverse the connections at the moment the induced elec- 
tromotive force changes its direction. This process is called 
commutation, and is accomplished by means of a commutator, 
whose segments are connected to the armature windings, 
and brushes which collect the current from the commu- 
tator. During the process of commutation, the armature 
conductors connected to the commutator segments covered 



122 DYNAMO ELECTRIC MACHINERY. 

by a brush are momentarily short-circuited. In this inter- 
val of time, the current flowing in a coil must be changed 
from a maximum value in one direction to zero, and from 
zero to a maximum value in the other direction. 

The current in an armature coil, at the beginning of a 
short-circuit by a brush, is responsible for the existence of 
a definite amount of magnetic flux which is linked with the 
turns of the coil. This flux would disappear if the current 
were suppressed, or would be built up in a reversed direc- 
tion upon the reversal of direction of the current. The 
product of this flux and the number of turns in the coil 
divided by the io 8 times the current in amperes flowing 
through the coil at the beginning of short-circuit, gives a 
quantity which may be termed the commutation self -induc- 
tance of the coil. Representing this quantity by L, the 
number of turns of the coil by n, and the number of mag- 
netic lines of force accompanying a current of I x amperes 
flowing in the coil by <l>, then the commutation self-induc- 
tance is 

L = henrys. 

The magnetic flux surrounding a coil carrying a current 
represents an amount of energy the magnitude of which is 
equal to \LI*. This energy must be disposed of and an 
equal amount oppositely directed must exist before the coil is 
in proper condition to be transferred from one side of the 
armature to the other at the brush. The magnetic energy 
may be dissipated either by the introduction of resistance 
to reduce the current flow or by a counter-electromotive 
force, which may be obtained by so placing the brushes 
that the short-circuited coil will cut some of the flux from 
that pole corner of the field magnet toward which the coil 



COMMUTATION. 123 

is moving. That is, the commutating plane is shifted so 
that, during the time a coil is short-circuited by a brush, 
the coil will traverse a magnetic field of opposite direction 
having such intensity as to produce an E.M.F. in it sufficient 
to reverse the direction of the initial current. If, during 
the time of short-circuit, the intensity of the current after 
reversal be identical with its original intensity, then spark- 
less commutation will ensue with this particular brush set- 
ting. At any other position of the brushes, the magnitude 
of the induced E.M.F. will be such as to result in a larger 
or smaller current value in the coil after reversal than its 
initial value, consequently sparking will occur during the 
process of commutation at such positions of the brushes. 
The greater the current output of a machine the greater 
must be the induced E.M.F. to effect current reversal, and 
consequently the field in which the coil is situated during 
commutation must be more intense, and therefore the brush 
position should change with the load. The present practice, 
however, is to have a definite brush position which is the 
same at all loads upon the machine. Therefore the com- 
mutating plane should be shifted forward in generators until 
no sparking occurs in the final position at full-load and also 
at no-load operation. 

There are thus two E.M.F.'s active in producing a current 
through the short-circuited armature coil: first, the electro- 
motive force induced in the coil by cutting the magnetic 
lines of force set up by the currents in the field magnet 
coils, and second, the E.M.F. occasioned by the varying 
flux due to the changing current in the coil. The latter 
E.M.F. is independent of any action of the field-magnets, 
and is called an electromotive force of self-induction or re- 
actance voltage, and it tends to prevent any increase or de- 



124 



DYNAMO ELECTRIC MACHINERY. 



crease in the strength of the current flowing. Sparkless 
commutation is dependent upon the magnitude of the re- 
sultant E.M.F. of these two opposing components. Were 
the resultant E.M.F. zero at every instant, perfect commu- 
tation might be obtained. But this ideal condition cannot 
be realized, since the component electromotive forces con- 
stitute different time functions. The induced E.M.F. de- 
pends upon the intensity of the field and speed of the 
armature, whereas the E.M.F. of self-induction depends 
upon the rate of current change in the coil. An approach 
to perfect commutation, therefore, is obtained by an adjust- 
ment of conditions whereby a number of instantaneous 
values of these opposing E.M.F.'s are equal during the 
time the coil is short-circuited by the brush or while it 
undergoes commutation. 

The current reversal in the armature coils is accelerated 
by the use of high-resistance brushes because the initial 
current is more quickly reduced to zero. Consider a coil 
of low resistance to be short-circuited by a high-resistance 
carbon brush, as shown in Fig. yy, the brush having the 
same width as the commutator seg- 
ments. Let i x and i 2 be the currents 
flowing in the taps to segments I 
and 2 respectively, and let I x be the 
current flowing through each arma- 
ture path between brushes. At the 
instant when commutator segment 
i is completely under a brush, the 
currents from both sides of the 
armature unite and pass through the corresponding tap; 
then 

i 1 =2l t and i 2 = o. 




Fig. 77- 



COMMUTATION. 125 

A moment later, the brush will be in contact with both 
segments ; then, if P be the short-circuited current, 

i x = I 1 + P and i 2 = I x — V. 

In this position, the high transition resistance between the 
brush and commutator will be less at segment 1 than that at 
segment 2, because of the greater contact area of the for- 
mer ; therefore the voltage drop across the contact at seg- 
ment 2 will be greater than that at the other segment. 
Because of this difference of voltage drops across the two 
segments, there is a tendency for a current to flow -in a 
direction opposite to the direction of t r This condition 
results in quicker reversal of the current. 

Continuing the cycle of commutation, a little later the 
brush contact area on both segments will be the same ; 
then, if the short-circuit current be zero, 

h = h = h- 

The next moment the direction of current P will be re- 
versed, and 

i 1 = I x — V and i 2 — I t + /'. 

Finally, when the brush rests only on segment 2, 

i t = o and i 2 = 2 I v 

The current density in the brushes varies considerably at 
different parts of it, and is not proportional to the E.M.F. 
because of the exceedingly variable transition resistance 
between brushes and commutator. This increases very 
rapidly if there be even minute sparking under the brushes. 

56. Time of Commutation. — The time interval during 
which the current changes from a maximum value in one 
direction to an equal value in the opposite direction is the 



126 DYNAMO ELECTRIC MACHINERY. 

time that elapses from the instant that one commutator 
segment reaches a brush to the instant the preceding seg- 
ment emerges from the other side of the brush. This time 
is evidently dependent upon the speed of the commutator 
and upon the width of the brushes, and is the time required 
for the strip of insulation between two successive segments 
to pass under the brush. If w b represent the breadth of 
the brush in inches, and m the thickness of mica between 
adjacent commutator segments in inches, then the time of 
the short-circuit is 

(Wb — m) , 

t c = seconds, 

v 

where v is the peripheral velocity of the commutator in 
inches per second. Representing the commutator diameter 
in inches by D ci and the number of revolutions per minute 
of the armature by V, then 

tzDcV 

v = — 

60 

Therefore 

_ 60 (wb — m) 

c ~ TlDcV 

The reciprocal of this time gives the number of commu- 
tations per second, or what is termed the frequency of com- 
mutation. The frequencies found in practice lie between 
200 and 800 per second. The breadth of a brush may be 
equal to the width of a commutator segment, but usually 
the brush is sufficiently broad to bridge over several seg- 
ments. For a definite brush width, the frequency of com- 
mutation is much higher for multiplex windings than for 
simplex windings. 



COMMUTATION. 



127 



Let + / be the current flowing in a coil just entering 
under a brush. After the time t Ci the current value in that 
coil for perfect commutation should be —I v What the in- 
stantaneous values of the current during this short interval 
of time t c will be, or how they may vary in a particular 
machine under certain conditions, cannot be foretold, yet 
Fig. 78 shows a possible time variation of the current in a 




Fig. 78. 



Fig. 79. 



coil undergoing commutation. It represents a sinusoidal 
current change. Numerous curves, such as Fig. 79, have 
been obtained showing the actual current variation during 
the period of short-circuit based upon experimental obser- 
vations, but such curves differ widely among themselves, 
and consequently no one of them may be taken to represent 
the general conditions occurring during commutation. 

57. Calculation of Reactance Voltage. — The reactance 
voltage of a short-circuited armature coil is occasioned by 
the varying flux which surrounds that coil when the current 
in it is changing. Its value depends upon the time rate 



128 DYNAMO ELECTRIC MACHINERY. 

of current change, and its instantaneous value may be ex- 
pressed as 

E/=-Lf, 
dt 

where L is the commutation self -inductance of the coil and 
P is the value of the current at the instant / seconds after 
the beginning of the short-circuit. It cannot be predeter- 
mined exactly how the successive values of P are related to 
each other during this interval, and it becomes necessary, 
in order to estimate the reactance voltage, to assume that 
the values of the short-circuit current follow some simple 
law. It is usual to assume a simple harmonic variation, as 
shown between the dotted lines of Fig. 78, in which case 
the instantaneous value of the current may be expressed as 

V = J, cos 2 n — t, 

1 2t c ' 

where I x is the current per armature path, and t c is the 

time of short-circuit (there are — complete variations per 

2 t c 

second). By substitution 

/( /iC0S f) 



whence 



E/ = 

dt 



E/=LI^ sin-^ 

vr. It. 



The maximum value of E s occurs when — is 90 ; then the 
reactance voltage of the short-circuited coil is 

E,=LlZ- (1) 

tc. 



COMMUTATION. 



129 



The time of commutation, t c , was obtained in the foregoing 
article ; thus for the determination of the reactance voltage 
there still remains the calculation of the self-inductance of 
the coil. 

To calculate the magnitude of L for a short-circuited 
armature coil or element, consider each coil side to lie in a 
slot, a typical form of which is shown in Fig. 80. The flux, 




Fig. 80. 



which is due to the current flowing through the n turns of 
this element, may take a number of paths across the slot, 
as shown by the numbered lines. Some lines of force also 
encircle the coil where it projects beyond the slots. The 
total inductance of a coil will be the sum of the inductances 
due to the flux through these various paths linking with the 
turns of the coil. All dimensions shown in the diagram 
will be expressed in inches. 

Consider an element of the conductors in the slot dx 



130 DYNAMO ELECTRIC MACHINERY. 

wide and at a distance x from the bottom of the slot. The 
magnetomotive force which produces the flux in this element, 

X 

due to the current I amperes flowing in -of the n con- 

a 

ductors of this coil side, is 

M.M.F = 4 Til" n ] - 1 - gilberts. 
\a / 10 ° 

Since the permeability of the iron is very much greater than 
that of the air, the reluctance of the iron portion of the flux 
paths may be neglected. Then the reluctance of the ele- 

mentarv path through the coil itself is d (R, = 

1 2.54 l a dx 

oersteds, where l a is the gross axial length of the armature 

core in inches. Hence the flux through this small area in 

maxwells is 

x L 

471-TI-+ 

,, a 10 _ 4 7r 2.54 nljaxdx 

1 w* 10 aw* 



2.54 l a dx 

x 
These lines of force are linked with — n turns, and there- 

a 

fore the elementary inductance in henry s, being io 8 times 
the number of linkages per ampere, is 

dL t = 4712.54 n ia x 2 dx. 
a 2 w s I 1 io 9 

Integrating over the full width of the coil, the inductance 
due to the flux through path 1 in henrys is 

n 2 l„a 



L i = 3i-9 2 9 / *?dx = 10.63 — 



W s IO 



Above the upper surface of the conductors, the magne- 
tomotive force is constant, and the lines of force through the 



COMMUTATION. 131 

upper portions of the slot are linked with all the conductors 

of the coil-side. The reluctance of path 2 is i — and the 

2.54/^ 

M.M.F. sending the flux through it is 4 tt/2 — ^ ; therefore 

the flux is 

= 4JLM 4 . nlM maxweUs _ 
2 10 w s 

The inductance due to this flux is 

n 2 l a b 
w s io 9 

Similarly the inductances due to the magnetic flux through 
paths 3 and 4 linking with the n turns of the coil are 

L, = 63.87 nHa \ 9 

3 (w s -f w )io 9 

and 

w IO 9 

For two surfaces, w inches apart, in the same plane, 
the paths of the magnetic lines of force may be taken as 
quadrants joined by straight lines of length w Q . Represent- 
ing the width of a tooth at the air-gap by / , and consider- 
ing only the flux outside the slot between two adjacent teeth, 
then the reluctance of the mean path 5 is 

W n + 7T- 



2 

&* = : — oersteds. 

2.54/^ 



There is additional flux, also occasioned by the current in 
the coil under consideration, which passes through adjacent 
teeth up to the limit of the interpolar gap. Because of its 
lesser influence, it will here be neglected. It is approxi- 



132 DYNAMO ELECTRIC MACHINERY. 

mately compensated for by the overestimation of the induc- 
tance of the coil as thus far calculated, due to neglecting 
the reduction of effective iron area occasioned by the pres- 
ence of air ducts and insulation between laminations. The 
flux passing through path 5 is 



3> = 



4712.54 nl l lj 1 



10 / 

2 



hence the resulting inductance is 



L,= 3i-9 



n 2 l„t 



a"! 



(w n + 71 



(^o + 4 



IO y 



Therefore the total inductance of the embedded portion of 
one coil-side, being the sum of the terms L x to Z 5 , in hen- 
rys, is 



L e = 10.63 n% 



3b , 6c 3d 3*1 



w s w s + w n W n , Tit, 



w r 



2 



(2) 



To obtain the inductance of a complete winding element, 
i.e., two coil-sides, twice this value L e must be taken, and 
to it must be added the inductance of the end-connections, 
or parts of the coil extending beyond the slots. It is usual 
to assume a magnetic flux of 2 maxwells per ampere-inch 
of conductor as linking with the exposed portion of a wind- 
ing element. The length of the end connection at one end 
of the armature core in machines of the usual type may be 
taken as 1.5 times the pole pitch, or 1.5^, inches. The 
inductance of the "free" portion of an armature coil of 
n turns would then be 

IO 8 ^ 



COMMUTATION. 



133 



Generally, two or more armature coils are simultaneously 
undergoing commutation, and since the coil-sides of two or 
more elements are usually in the same or adjacent slots, 
there is a mutual inductive action between them. In Fig. 
81, one coil-side of the winding element, short-circuited by 




the positive brush, lies in the same slot with one coil-side 
of the element short-circuited by the negative brush. The 
mutual inductance with such an arrangement is very nearly 
equal to the inductance of the embedded portion of a coil. 
Thus little error will be introduced by taking double the 
value of L e , as previously calculated, to include the effect 
of mutual inductance. Since the end connections of the 
two coils of Fig. 81 do not coincide but are widely sepa- 
rated, no mutual inductive action between these portions of 
the winding elements need be considered. 

The total inductance of a short-circuited coil is therefore 

L = 4L e +L f . (4) 

The values of t ci I t , and L now being known, the reactance 



134 DYNAMO ELECTRIC MACHINERY. 

voltage of a coil undergoing commutation may be deter- 
mined from equation (i). 

In dynamos having lap-wound armatures the above value 
of L would be that for a coil between two adjacent com- 
mutator segments, or, as sometimes stated, the inductance 
per segment. In dynamos having wave-wound armatures 
with only two brushes, the foregoing value of coil induc- 
tance is that for one winding element. For one coil of 
this type of armature with p elements terminating at two 
successive commutator segments, the inductance would 
be p times as great. Consequently the employment of as 
many brushes as there are poles is desirable from the 
commutation viewpoint. 

A quick method of estimating the inductance of the 
embedded portion of an armature element, due to Hobart, 
is based upon the assumption that a flux of 10 maxwells 
surrounds each inch of conductor length per ampere of 
current which flows through it. Then 

(iol nl n )n 97 7 . , . 

L e = v 1/~ = nln IO henrys, (5) 

where l n is the net axial length of the armature core. 
Combining with equation (3), the total inductance of a 
short-circuited coil in henrys is 

L = n 2 (40l n + 6X p )io- 8 ' (6) 

Except for the large slow-speed dynamos, the reactance 
voltage should not exceed 2 volts per segment in order to 
obtain fair commutation. For these large machines the 
value of the reactance voltage may reach 5 volts, but the 
aim of the designer is to obtain a lower value. 

58. Conditions for Good Commutation. — There are two 
E.M.F.'s active in producing a current through an arma- 



COMMUTATION. 135 

ture coil undergoing commutation, namely, (1) the electro- 

dP 

motive force of self-induction, which is equal to — L — -, 

dt 

and (2) the electromotive force induced in the coil due to 
its cutting lines of force, the value of which may be ex- 
pressed as some function of the time, or /(/). To com- 
plete the electromotive force equation of a short-circuited 
coil, the various resistance drops must be inserted. Let 
R, Fig. 77, be the resistance of the coil undergoing com- 
mutation, and R c be the resistance of each connection tap 
to the commutator. Then the resistance drop across two 
adjacent segments (§55) is 

PR + i.Rci- i 2 R c = I'R + Rc [I, + I f + I, - P] = I'R + 2 I X R C . 
To include the voltage drop from the brushes to the 
commutator, let R t be the transition resistance of one set 
of brushes, having a breadth equal to the width of a com- 
mutator segment, when resting on only one segment. At 
the time / seconds after the beginning of the short-circuit, 

the transition resistance of segment 1, Fig. 77, is — - — R v 

i c z 

and that of segment 2 is — R r The corresponding resist- 
ance drops are respectively 

tc — t tc t 

and 

h Ri i 2= h^ {Ii _ r) . 

The complete E.M.F. equation of a coil undergoing spark- 
less commutation may then be expressed as 



m -L^-VR- 2l lRc - ^[JJl±£ + L_^ = 



O. 



136 DYNAMO ELECTRIC MACHINERY. 

An analysis of this equation has shown that for spark- 

less commutation in the neutral plane - 1 t c must be equal 

to or greater than unity. This implies that the transition re- 
sistance of brushes should be great, that the inductance of 
the short-circuited coil should be small, and that the time 
of commutation be comparatively large. As the width of 
the brushes is generally such as to short-circuit several coils 
simultaneously, the above general equation must be modi- 

fied accordingly. However, —- 1 t c remains practically un- 

altered, since both R x and L are multiplied by the number 
of coils simultaneously short-circuited by one brush. 

Because of the great transition resistance between cop- 
per commutator segments and carbon brushes, these are 
more generally used than copper brushes, although the latter 
find application in high-speed turbo-generators as well as 
in low-voltage generators or motors. For a fixed brush 
position at all loads there is a tendency to spark when the 
machine is subjected to wide variations of load, but the use 
of carbon brushes counteracts this tendency to a great extent 
because of their high transition resistance. With copper 
brushes, a fixed brush position for all loads can rarely be 

attained. The value of — 1 t c usually exceeds 2 with car- 

J-* 

bon brushes, but may occasionally fall below \ for copper 
brushes. 

As the inductance of a short-circuited coil depends upon 
the square of the number of turns, it is desirable to have 
few turns per coil, so that the reactance voltage may be low. 
Good practice limits the number of turns of a coil in moder- 
ate-sized machines to 2 or 3. An inspection of equation (2) 



COMMUTATION. 137 

of § 57 shows the desirability of having the axial length of 
armature core small so as to reduce L e ; this implies that the 
armature diameter be large for the same output. Large 
diameters permit of the employment of large commutators 
having many segments, consequently there will be fewer 
armature turns per segment than would be possible with 
smaller commutators having bars of equal width ; this con- 
dition, as already stated, is conducive to a lower reactance 
voltage. 

The time of commutation could be raised by increasing 
the width of the brush, but if the brush bridges over more 
than a few segments, the inductance of each short-circuited 
coil will be much greater because of its turns linking with 
lines of force produced by the current in other coils as well. 
A further limitation to increasing the time of short-circuit 
by the use of wide brushes is the lowering of the tran- 
sition resistance R x due to the greater contact area of the 
brush. 

There are a number of purely mechanical conditions upon 
which depend the quality of commutation. A rough commu- 
tator surface causes vibration of the brushes, which results 
in a widely varying transition resistance and subsequent 
sparking. A further source of sparking, more especially in 
high-speed machines, is loose commutator bars. Such spark- 
ing produces local blackening of the commutator surface. 
An open or discontinuous armature winding and a reversed 
coil occasion severe sparking. The limit of the capacity of 
a machine may be excessive sparking instead of excessive 
heating, and therefore the suppression of sparking by proper 
design of the machine is of utmost importance. 



138 DYNAMO ELECTRIC MACHINERY. 



PROBLEMS. 

1. Compute the field ampere-turns per pole necessary to com- 
pensate for demagnetization in a 15-K. W., 125-volt, 4-pole 
dynamo having a wave-wound armature, the winding being con- 
tained in 1 2 1 armature slots, with 4 conductors per slot. The 
commutator has 121 segments, and the commutating plane is 
shifted 3 segments ahead. Dispersion coefficient = 1.25. 

2. Calculate the field ampere-turns per pole necessary to neu- 
tralize the distortional effect of the armature current of a 6-pole 
generator with a triplex lap-wound armature having a total of 
1086 conductors. The rated current of the machine is 500 
amperes. The angle of brush lead is 8 degrees. No-load field 
ampere-turns per pole = 7200. 

3. For the dynamo of Prob. 4, Chap. IV, calculate the addi- 
tional ampere-turns required to compensate for armature reac- 
tion. The armature has a simplex wave winding with two con- 
ductors per slot ; the angle of brush lead is 5 degrees. 

4. Determine the time of short-circuit of an armature coil 
undergoing commutation of a 12-pole, 350-K. W., 250-volt, 70- 
rev.-per-min. generator having a commutator 56 inches in diam- 
eter with 448 segments. The brushes exactly cover two seg- 
ments with the intervening insulation, which is .03 inch thick. 

5. The generator armature of the foregoing problem has a 
simplex lap winding, and the inductance of a short-circuited 
coil is 0.000030 henry. Compute the reactance voltage per 
segment. 

6. Determine the reactance voltage per segment of a 12-pole 
300-K. W., 500-volt generator making 200 rev. per min., the 
constants of which follow : 

Diameter of armature =75 in. 

Gross length of armature =18 in. 

Diameter of commutator =50 in. 



PROBLEMS. 



139 



Number of commutator segments = 489. 

Thickness of insulation between segments = 0.04 in. 
Number of armature slots, as per Fig. 82 =489. 
Size of armature conductors = |"x J"- 

Armature is lap wound ; two conductors per slot. 
Brush width = I in. 



T 




a 



I" 

.JL 



Fig. 82. 



140 DYNAMO ELECTRIC MACHINERY. 



CHAPTER VI. 

GENERATORS. 

Efficiency of Operation. 

59. Capacity of a Dynamo. — The capacity of a genera- 
tor is measured by the power it can develop, that is, the 
capacity varies as the product of the terminal electromo- 
tive force and the current supplied to an outside circuit. 
The E.M.F. of a dynamo depends upon the speed, the 
number of conductors, and the number of magnetic lines 
of force passing through the armature, § 39. The allow- 
able current output depends primarily upon the size of the 
armature conductors, so that these may carry the required 
current without excessive heating. The larger the con- 
ductors, the larger must be the armature core, other things 
remaining the same. Sometimes, however, commutation 
difficulties limit the output of a machine rather than tem- 
perature elevation. 

The E.M.F. of a generator may be raised by increasing 
the speed, the number of conductors, or the magnetic flux 
through the armature. The speed of a machine is limited 
by considerations of mechanical strength and economy of 
material. It is frequently specified in that the generator 
is to be directly connected to a steam engine, turbine, or 
other prime mover, or, in the case of a motor, by direct- 
coupling to the machine it operates. The speed of small 
machines is greater than that of large ones, but the periph- 
eral velocity for nearly all sizes lies between 25 and 100 
feet per second for belt-driven machines, and between 25 



GENERATORS. 



141 



and 50 feet per second for direct-connected machines. In 
turbo-generators the speeds may be as high as 250 feet 
per second. The speed limits of modern direct-current gen- 
erators, in revolutions per minute, are given in the follow- 
ing table : 



K.VV. 


GENERATOR SPEEDS. 


DIRECT-CONNECTED . 


BELT-DRIVEN. 


5 

10 
20 

5° 
100 

200 

500 

TOOO 
150O 
2000 


400—800 

350-500 

250-400 

I80-350 

120-300 

IOO-25O 

70-I20 

60-9O 

55^5 

50-80 


65O-2OOO 

600-l800 

550-I600 

500-I200 

450-900 

4OO—60O 

3CO-400 



The number of inductors on a given armature can be 
increased by decreasing the size of the wire. Sufficient 
cross-section must, however, be provided in the conductors 
to enable them to carry the maximum current of the 
machine without causing them to heat to such an extent 
as to endanger the insulation. Good practice calls for from 
400 to 800 circular mils for armature conductor cross-sec- 
tion per ampere, the proper value depending upon the heat- 
dissipating facilities in the machine. The smaller values 
are suitable for intermittently operating machines, such as 
elevator or crane motors ; whereas the larger values are for 
continuously running machines, such as central-station 
generators. 

The field flux of a generator, for a path of constant reluc- 
tance, depends upon the magnetomotive force produced by 
the current in the field winding. Increasing the number 



142 



DYNAMO ELECTRIC MACHINERY. 



of turns on the field coils or raising the current flowing in 
them would increase the magnetic flux passing through the 
armature. Decreasing the reluctance of the path of the 
flux yields a similar result ; hence the desirability of small 
air-gaps, magnetic material of high permeability, and short 
flux paths of large cross-section. 

Because of armature reaction, it is desirable to limit 
the current flowing in the armature conductors. This is 
accomplished by providing numerous paths between brushes 
for the armature current, so that only a small portion of 
the total current flows through each coil. This implies the 
provision of a suitable number of field poles, which is there- 
fore a necessary condition for obtaining satisfactory oper- 
ation as regards sparking. The usual limits as to the 
number of poles on commercial direct-current generators 
are given below : 



K.W. 


NUMBER OF POLES. 


I-I5 


2-4 


I5-IOO 


4-6 


IOO-200 


6-8 


200-300 


6-IO 


300-500 


8-12 


50O-IO0O 


10-16 


rOOO-2000 


12-24 



60. Heating of Dynamos. — When a generator delivers 
current, there is a continuous production of heat in the 
armature and field magnets, which represents the conver- 
sion of some electrical energy into heat. This production 
of heat is occasioned by eddy current and hysteresis losses 
in the iron, copper losses in both armature and field wind- 
ings, bearing friction and windage, pole-face losses, and 
commutator losses. The temperature of the machine, 



GENERATORS. 



143 



therefore, continually rises until a temperature is reached 
at which as much heat will escape per unit of time as is 
generated in an equal period. The dissipation of heat 
takes place by conduction, air convection, and radiation. 
The ultimate temperature of any part of an operating 
machine depends upon the emissivity and area of the radi- 
ating surface and its temperature elevation over the sur- 
rounding atmosphere. Hence it is necessary to design each 
part of the dynamo so that its temperature rise during 
continuous full-load operation shall not exceed a certain 
prescribed limit. 

The ultimate constant temperature is usually acquired 
after from 6 to 18 hours of full-load operation, accord- 
ing to the size and construction of the machine. The 
temperature of the armature of a 300-K.W. generator 
operating under constant full load, in terms of time, is 

























60 

50 

z 
















































13 

< 
DC 

£ 30 

£ 

20 
10 



















































































HOURS 

Fig. 83. 



shown in Fig. S$. It is obviously possible to obtain a 
greater output for a short time without excessive rise of 
temperature ; for example, a machine may yield 25 per cent 



144 



DYNAMO ELECTRIC MACHINERY. 



overload capacity for two hours without undue temperature 
elevation. Fig. 84 shows the ultimate temperature of the 
armature of the same 300-K.W. generator under different 
loads, a constant temperature being attained at each load. 



125 

100 
75 
50 
25 



































































































150 300 450 

LOAD IN KILOWATTS 



Fig. 84. 

Two methods for obtaining the rise of temperature are 
in common use : a, by a thermometer ; b, by increase of 
electrical resistance. The latter method is to be preferred, 
and should be used wherever practicable. In taking the tem- 
perature of a surface of the machine by the first method, 
the thermometer bulb should be laid flat against that sur- 
face and be covered by a pad of cotton sufficiently small to 
allow normal escape of heat from the surface. The use of 
the cotton pad prevents radiation of heat from the bulb of 
the thermometer. The determination of the temperature 
rise of a winding by the second method involves the meas- 
urement of its resistance at room temperature and at the 
ultimate temperature assumed under full-load operation. 
Knowing the temperature coefficient of resistance of the 
material at o° C, the rise in temperature may be com- 
puted, § 4. 

The temperature elevation of a part of a machine as 



GENERATORS. 145 

determined thermometrically by applying a thermometer to 
the hottest accessible portion, may frequently be less than 
70 % of the temperature rise as computed from the re- 
sistance measurements, this difference depending upon 
the construction of the part under test. If, however, a 
thermometer applied to a winding indicates a higher tem- 
perature elevation than that obtained from resistance 
measurements, then the thermometer indication should be 
accepted. 

At rated load and under normal conditions of ventilation, 
the maximum temperature rise, referred to a standard room 
temperature of 25 C, should not exceed 50 C. for field 
coils and armature, as measured by resistance increase ; 
55° C. for commutator and brushes, and 40 C. for bearings 
and other parts of the machine, as thermometrically deter- 
mined. 

61. Output Coefficients. — The capacity or rating of a 
machine depends to a great extent upon its heating. In 
the armature, the conductor cross-section must be such 
that the full-load current may flow through the winding 
without producing an undue temperature rise. The em- 
ployment of large conductors requires large armatures. 
Therefore an approximate estimate of the capacity of a 
machine can be obtained if the dimensions and speed of 
the armature be known. 

An empirical expression, given by Kapp, for the rated 
output of a generator in kilowatts is 

P = w%v, 

where $ = a factor called the output coefficient, 

D — armature diameter in inches, 

l a = gross axial length of armature core in inches, 
and V = rev. per min. of the armature. 



146 



DYNAMO ELECTRIC MACHINERY. 



The value of the output coefficient depends upon the arma- 
ture diameter, and may be obtained from the curve of Fig. 
85, which yields results typical of good practice. The fore- 
going equation is useful 
for rough preliminary de- 
sign purposes, the decision 
as to final dimensions 
being subject to consider- 
ations of commutation 
and voltage regulation. 

62. Losses in Armature 
Cores. — The loss of en- 
ergy which attends the 
rotation of an armature 
in a magnetic field is due 
to hysteresis and eddy 
currents in the armature 
core. In order to reduce 
these losses, and thereby 
to prevent an excessive 
temperature rise, arma- 
ture cores are composed 
of a series of thin disks 
or laminations, insulated 
more or less thoroughly 
from each other. The magnitudes of the hysteresis and 
eddy current losses, as shown in §§ 28 and 29, depend 
upon the size of the core, the magnetic flux density 
in its various parts, the thickness of the laminations, 
and the number of magnetic cycles passed through per 
second by the iron. Curves of hysteresis and eddy cur- 
rent losses in armature cores, in watts per cubic inch 



6 ^— * ^= 

5 -I 

^ 4 — L— _ 

' 3~ 

II 



50 100 150 

ARMATURE DIAMETER IN INCHES 



Fig. 85. 



GENERATORS. 



147 



and per pound, expressed in terms of flux density, are given 
respectively in Figs. 86 and 87, for both 25 and 60 cycles 



3.0 



Q 2.5 

z 

O2.0 
a. 

£l.5 
a. 

[2 1.0 

< 
£0.5 



1.0 











1 


















H 


YST 


ERE 


SIS 


LOS! 










#X 






















6© 


tf*2 


















































J£~ 


c^X^- 


































84 



70 



.56 



42 



28 < 
.14 



23 45 6 78 910 
KILO-MAXWELLS PER SQ. CM. 
Fig. 86. 



11 12 



a. 
co .4 

h 

I- 2 





























-.so . 

2 

-.24" 




ED 


DY ( 


)UR 


3EN 


T L( 


)SS 














-.20 


















n,C 


vcv-5 


s^- 






"•MS 


















6©^^- 








.12 |_ 
-.08< 
















25 C 


YCLES 

1 








-.04 * 



4 5 6 7 8 9 10 
KILO-MAXWELLS PER SQ. CM. 
Fig. 87. 



11 12 



per second. These curves are plotted from values calcu- 
lated by means of the formulae 

P*-8. 3 ^(Blfio- f 

P e = 4 .o7Tf/ 2 <io- 17 , 

where v = volume of iron in cubic inches, 
f = cycles per second, 

/ = thickness of laminations in mils (usually 14), 
(B m = max. flux density in maxwells per sq. in., 
and 7] = hysteretic constant (taken as 0.0021). 



148 DYNAMO ELECTRIC MACHINERY. 

63. Armature Copper Loss. — When a generator deliv- 
ers current to an external circuit, this current, in flowing 
through the armature winding, occasions a loss of energy, 
which appears as heat. The magnitude of this loss at any 
load is equal to the product of the armature resistance from 
the positive to the negative brushes (excluding brush tran- 
sition resistance) and the square of the total current of the 
machine at the definite load. If 5 be the number of con- 
ductors in series between brushes, that is, the total number 
of conductors on the armature divided by q (= number of 
current paths through armature from positive to negative 
brushes), if L a be the length in inches of the embedded 
portion of one conductor plus the length on one end of the 
exposed or free portion, and if A a be the cross-section of 
an armature conductor in square inches, then the total 
resistance of the armature in ohms is 

0.825 LaS 

where the constant 0.825 X io -6 is the resistance in ohms 
between opposite faces of an inch cube of copper at 75 C. 
Values of q for various types of armature windings are 
given in the table of § 39. 

For preliminary design purposes, in order to save time, 
the value of L a is frequently taken as the sum of the 
embedded length of a conductor, l at plus 1.5 times the 
pole pitch. That is, the quantity 1.5 X p is an estimation of 
the free length of an armature conductor on one end of 
the core. Then 

L a =la+ 1.5 V 

The armature copper loss at full load in watts is there- 
fore p _ pR 



GENERATORS. 149 

where I is the full load current of the dynamo leaving or 
entering at the brushes. 

When large armature conductors pass through a non- 
uniform magnetic field, such as exists under the field-pole 
corners, eddy currents will be produced in them because 
of the greater E.M.F. induced in one side of the conductor. 
This loss is reduced in large machines by using several 
conductors connected in parallel instead of one large equiva- 
lent conductor. 

64. Pole-Face Losses. — In dynamos having toothed 
armatures the reluctance of the air-gap between the arma- 
ture and the field poles is less opposite a tooth than oppo- 
site a slot. Consequently more lines of force pass through 
portions of the pole face opposite armature teeth than 
through those portions opposite the slots in the armature 
core. As the armature rotates each point of the pole face 
is subjected to a pulsating flux, thus giving rise to eddy- 
current and hysteresis losses in the pole pieces. To min- 
imize this effect pole pieces are often constructed of 
laminated iron or sheet steel. 

The magnetomotive force of the eddy currents tends to 
equalize the flux density, and therefore the pulsation is 
confined to a very thin surface layer of the pole pieces. 
An expression for the pole-face loss of a dynamo in watts, 
given by Adams, is 

where (B is the average pole-face flux density in maxwells 
per square inch ; A p is the total pole-face area of the 
dynamo in square inches ; v is the peripheral velocity of 
the armature in feet per second ; p is the permeability of 
the pole face ; p is the electrical resistivity of the pole face 



i5o 



DYNAMO ELECTRIC MACHINERY. 



in c.g.s. units (p equals about 1500); k is 1.6 times the 
square root of the tooth pitch in inches for solid pole pieces, 
and 4.1 times the thickness of the laminations in inches 
divided by the square root of the tooth pitch in inches for 
laminated pole shoes ; k' is a constant depending upon the 
ratio of slot opening, w , to the radial length of the air- 
gap, A, and may be taken from Fig. 88. The foregoing 
expression has received experimental verification. 





















0.6 




































0.4 


































0.2 





















































A 
Fig. 88. 



The pole-face loss of a dynamo is usually less than one 
per cent of the output of the machine. There is another 
source of energy loss in the pole face which is due to re- 
luctance pulsation of the magnetic circuit of the machine 
occasioned by a variation in the number of teeth under a 
field pole. Large air-gaps and chamfered pole corners 
practically eliminate the flux pulsation. In the case of 
laminated pole pieces, there are some additional losses due 



GENERATORS. I 5 I 

to transverse bolts and screw heads which serve to hold 
such shoes on the field cores. 

65. Excitation Loss. — The loss of power due to the 
current flowing through the field-magnet windings for pro- 
ducing the magnetic field of the machine is easily calcu- 
lated as the product of the square of that current and the 
resistance of the winding. Thus, the excitation loss of a 
series-wound dynamo in watts is 

Pf=Ise 2 Rse> (I) 

and that of a shunt-wound machine in watts is 

Pf = i S k 2 Rsk = |^ ; (2) 

where I se and I sh in amperes are the currents flowing in 
the series and shunt field coils respectively, R se and R sh 
are the resistances thereof in ohms at the steady running 
temperature (§ 49), and E is the terminal voltage of the 
generator. In a compound-wound machine the total excita- 
tion loss is the sum of the foregoing expressions, or 

Pf = Ise 2 Rse + Ish Rsh- (3) 

Care must be exercised to apply this equation correctly for 
long-shunt and short-shunt compound-wound dynamos. 

With separately excited field magnets the power loss in 
the resistance of the field-magnet coils alone should be con- 
sidered, but with either shunt- or series-wound field coils the 
power loss in the accompanying regulating rheostat should 
also be included, since this apparatus is considered an 
essential part of the machine. 

66. Bearing Friction and Windage. — As an armature 
revolves, some energy is wasted in bearing friction and 
windage, and this loss may be considered independent of 



152 



DYNAMO ELECTRIC MACHINERY. 



the load on the machine. Its value cannot be determined 
accurately, but may be estimated by means of the curve 
in Fig. 89, given by Hobart. The loss in watts due to 



i6co 



*fv> 

















































































V = 


1000 

































D 2 ( l a +.7X p ) 

Fig. 89. 

friction and windage, P fw , is plotted against the product 
of the square of the armature diameter in inches into the 
axial length of the armature over the end connections of 
the winding. The latter factor may be considered as the 
gross length of the armature plus seven-tenths of the pole 
pitch, or 4 + 0.7 X p inches. The curve refers to a speed 
of 1000 rev. per min. ; the friction and windage loss at 
any other speed is taken in direct proportion. 

This loss is usually less than one-half per cent in ma- 
chines of over 1000 K. W. output, and may be 2 to 3 per 
cent in machines of 20 K. W. or under. 

67. Commutator Loss. — The transition resistance be- 
tween the brushes and commutator causes a drop in volt- 
age at each point of contact, the magnitude of which 



GENERATORS. 1 53 

depends upon the quality of the brush, but is practically 
independent of commutator speed, brush current density, 
and brush pressure, § 42. This drop for both positive and 
negative brushes varies between 1.2 and 2.8 volts. There- 
fore the product of this drop times the current leaving or 
entering at the brushes gives the loss due to the brush 
transition resistance. 

The pressure of the brushes on the commutator causes 
a friction loss. This quantity may be expressed as equal to 

— — watts, § 4 2 

33000 X 12 

where D c = commutator diameter in inches, 

V = rev. per. min. of armature, 

p! = coefficient of friction (0.30 for carbon brushes 
and 0.25 for copper brushes), 

F = sum of pressures of all brushes on commuta- 
tor in pounds ; generally 1.25 lbs. per sq. in. 
of rubbing surface. 

To allow for the additional loss at the commutator due to 
sparking at the brushes and currents in the short-circuited 
segments, which cannot be determined accurately, six per 
cent is usually added to the regular commutator loss. 
Therefore the total commutator loss in watts may be ex- 
pressed as 

P c = 1.06 [(1.2 to 2.8) I + 0.0059 DcVfi'F]. 

68. Temperature Elevation. — The temperature eleva- 
tion of any part of a dynamo is proportional to the watts 
expended in that part and inversely proportional to its 
radiating surface. The rise of temperature will be influ- 
enced considerably by the speed of the armature and by 



154 DYNAMO ELECTRIC MACHINERY. 

the effectiveness of the ventilating arrangements. The 
temperature rise is considered separately for armature, 
field coils, and for commutator. 

The total losses in the armature comprise the eddy- 
current and hysteresis losses in the core and the copper 
loss in the winding. The true radiating surface of the 
armature is difficult of determination, since the end con- 
nections of the winding and the surfaces on both sides of 
the ventilating ducts assist in radiating some of the heat 
developed in the armature. It is more convenient to con- 
sider a surface to which the cooling effect may be regarded 
as approximately proportional, and such is the external cylin- 
drical stirface of the armature. 

For well -ventilated armatures of modern dynamos, the 
temperature elevation in degrees Centigrade as thermo- 
metrically measured may be obtained from the following 
expression due to Arnold : 

T HP e + p h + p a ) 

1 a — 



7zD(l a + 0.7 X p ) (1 + .031/) 

where P e + P h = core losses in watts, § 62, 

P a = armature copper loss in watts, § 63, 
D = armature diameter in inches, 
la + 0.7 X p = axial length of armature over end con- 
nections in inches, § 66, 
v = peripheral velocity of armature in feet 
per second, 
and k x = a constant the value of which may be 

taken as 55. 

The temperature rise of the field coils depends upon the 
depth of the winding, the heat emissivity of the bobbins 



GENERATORS. 155 

upon which the wire is wound, and the effect of the 
fanning action of the revolving armature. For multipolar 
machines of modern design, the temperature rise in degrees 
Centigrade, as obtained from resistance measurements, may 
be calculated from the following expression also given by 
Arnold, 

where Pj is the total excitation loss of the dynamo in 
watts, § 65, Af is the area of the exposed surface of all 
the field coils in square inches, and k 2 is a constant the 
value of which may be taken as 90. 

For the temperature rise of commutators, the same 
authority gives the following empirical equation : 

k P 



where T c = temperature elevation of the commutator in 
degrees Centigrade, 
P c = commutator loss in watts, § 67, 
D c = commutator diameter in inches, 
4 = length of commutator in inches, 
v = peripheral velocity of commutator in feet per 
sec, 
and k 3 = a constant depending upon degree of ventila- 
tion ; its value may be taken as 20. 

69. Efficiency. — The efficiency of a machine is defined 
as the ratio of its net power output to its gross power 
input. It may also be defined as the ratio of the net 
power output to the sum of the net power output and the 
total losses. If P be the output in watts, and P in be the 



156 DYNAMO ELECTRIC MACHINERY, 

input to a dynamo in watts, then the efficiency is 

Po Po 



Pm Po + (p h + p e + p a + p P + p f + p fw + p c y 

the various losses being determined as in §§ 62 to 6j. 
The efficiency of a machine at full load should be deter- 
mined at the ultimate temperature assumed under con- 
tinuous operation at rated load, referred to the standard 
engine-room temperature of 25 C. 

The electrical power delivered by, or supplied to, a 
dynamo should be measured at the terminals of the 
machine, and is given by the product of the terminal volt- 
age and the ampere output. The mechanical power should 
be measured at the pulley, gearing, coupling, etc., thus 
excluding the losses in these devices, but including the 
bearing friction and windage. If, however, a generator be 
mounted directly upon the shaft of a prime mover so that 
it cannot be separated therefrom, the frictional losses in the 
bearings and in windage may be disregarded in determining 
the efficiency of the dynamo, owing to the difficulty in appor- 
tioning these losses between prime mover and generator. 

Where a machine has auxiliary apparatus, such as an 
exciter, the power lost in the auxiliary apparatus should not 
be charged to the machine, but to the plant consisting of 
machine and auxiliary apparatus taken together. In such 
cases plant efficiency should be distinguished from machine 
efficiency. 

The efficiency of a dynamo increases with the size, being 
low on small machines, and quite high on the larger ones. 
The efficiencies to be expected of modern direct-current 
compound- or shunt-wound generators of various sizes at 
full load are shown by the curve of Fig. 90. 



GENERATORS. 



157 




100 



































€ 






















































































































Pa y 
















p e +: 


'a 
















































Pf-sk 






S 


* 






















^ 


<r 








-**"^ r 








Pfw 












Pf-se- 












?p 












P C 



















100 



PERCENTAGE OUTPUT 

Fig. 91. 



The efficiency of a compound- or shunt-wound dynamo 
is small at low outputs because the practically constant 
core losses, friction and windage loss, and shunt-field exci- 
tation loss are then large in comparison with the power 
output. Fig. 91 shows how the efficiency of a certain 



158 DYNAMO ELECTRIC MACHINERY. 

200-K.W. compound-wound generator increases with the 
output. Curves are also given in this figure which show 
the variation of the different losses with the output of the 
generator. Since the distribution of the magnetic and 
electrical losses of a generator lies within the discretion of 
the designer, it is possible to so design a machine as to have 
its point of maximum efficiency at full load or at some other 
specified load. As a rule, however, the exact location of 
the maximum efficiency is hardly considered in designing 
a dynamo, since the efficiency near the maximum value is 
fairly constant over wide variations of load. 

70. Coefficient of Conversion. — The coefficient of con- 
version of a generator is the ratio of the total electrical 
energy developed in the armature winding to the total me- 
chanical energy supplied to the armature. This is some- 
times called the efficiency of conversion, but to distinguish 
it from efficiency as defined in the foregoing article, it is 
better to use the term coefficient of conversion. This co- 
efficient is always less than unity, and is expressed by 

F- T 

n J- in 1 



E in I + P e + Ph + P fw + P v 

where E in is the actual voltage generated in the armature, 
i.e., internal E.M.F., I is the current of the generator in 
amperes leaving or entering at the brushes, and P e , P h , 
P fw , and P p are respectively the eddy-current, hysteresis, 
friction and windage, and pole-face losses of the machine 
in watts. 

71. Economic Coefficient. — Some of the electrical power 
developed in a generator armature is consumed in overcom- 
ing the resistance of the armature winding, some is wasted 
at the commutator, and some is expended in exciting the 



GENERATORS. 159 

field magnets ; the remainder being delivered as useful 
power to the external circuit, or load. The ratio of this 
useful electrical energy to the total electrical energy devel- 
oped in the armature is known as the economic coefficient, 
and sometimes as the electrical efficiency. Hence, the eco- 
nomic coefficient may be expressed as 

EjJ ~ (P a +Pc+ Pf) 
7?= , 

where P a , P c , and Pf are respectively the armature copper 
loss, commutator loss, and excitation loss of the generator 
in watts. 

The efficiency, or, as it is sometimes called, commercial 
efficiency, of a generator is evidently the product of the con- 
version and economic coefficients, or 

e = fa. 

72. Magnetos. — Magnetos or magneto-generators are 
dynamos in which the magnetic flux is set up by perma- 
nent magnets. Since the flux density in this type of 
machine is necessarily low, for a given flux more iron must 
be used than in machines having their fields produced by 
electro-magnets. Therefore the application of magnetos 
is limited to purposes requiring a relatively small amount 
of energy, such as telephone signaling, automobile ignition 
work, and testing of electrical circuits. 

Magnetos are generally alternating-current machines and 
provided with slip rings or contact studs instead of com- 
mutators. The armatures are usually of the Siemens 
type, wound with many turns of fine wire, and mounted 
so that they may be rapidly rotated between the poles of 



160 DYNAMO ELECTRIC MACHINERY. 

permanent horseshoe magnets. Fig. 92 shows a telephone 
magneto-generator manufactured by the Western Electric 
Company. To the armature shaft is affixed a pinion which 
meshes with a gear wheel turned by hand. Such machines 
are designed to ring a call-bell or telephone ringer through 
an external resistance as high as 50,000 ohms. The 
armature windings of magnetos have resistances between 
300 and 600 ohms depending on the type of service for 
which they are designed. These generators are provided 




Fig. 92. 



with " shunts," which afford a by-path of low resistance 
around the armature when not in use, or devices which 
normally hold the armature circuits open and close them 
when the generators are operated. The air gaps of such 
machines may be as low as 0.0 1 inch without introducing 
operative difficulties. 

73. Constant-Potential and Constant-Current Supply. — 
There are in use two systems of electrical distribution : 
(a) at constant potential, (b) with constant current. In 
the former system, lamps, motors, or other types of elec- 



GENERATORS. l6l 

trical apparatus are connected in parallel with each other 
across the supply mains. To secure satisfactory operation, 
it is necessary to maintain a constant voltage between 
these mains, so that if some of the load be disconnected, 
or more load be added, the current flowing in the remain- 
ing lamps, motors, etc., will stay unchanged. This method 
of supplying, at any point of usage, current at a constant 
potential irrespective of the load which is there or at other 
points of the system, is very generally used in the distri- 
bution of electrical energy for incandescent electric light- 
ing, for operation of constant-pressure motors, and for 
electric traction. The great sensitiveness of the light in- 
tensity of incandescent lamps to a change in voltage, the 
candle power varying perhaps as the fourth power of the 
voltage, requires that the voltage across electric-lighting 
supply mains vary less than 3 per cent of its rated value. 
In electric traction, where the load is exceedingly variable, 
particularly in trunk-line operation, constant-potential dis- 
tribution can only be approximated. Frequently a drop 
of 25 per cent is allowed. 

For lighting by arc lights where considerable energy is 
expended at the points of illumination, and where these 
points are separated from each other by considerable dis- 
tances, it is sometimes economical and desirable to connect 
the lamps in series. For satisfactory operation the current 
in the circuit should be maintained constant, so that if 
more lamps be put in service, or some taken out, the volt- 
age across the remaining lamps will be unchanged. A 
lamp connected to such a system may be cut out by short- 
circuiting it. 

The advantage of constant-current distribution for town 
lighting is the economy of copper for the supply mains. 



162 DYNAMO ELECTRIC MACHINERY. 

The line can be made of much smaller wire than in the 
case of a constant-pressure circuit, for on a constant-current 
circuit as the load increases the power or energy trans- 
mitted is increased by raising the potential, the current 
remaining unaltered ; while in a constant-pressure circuit 
an increase of load is met by an increase of current, and 
the supply mains must be of sufficient size to safely carry 
the required maximum current. The size of wire neces- 
sary is dictated, not by the energy transmitted, but by the 
current flowing, hence a wire large enough to supply just 
one lamp of a constant-pressure circuit can supply all the 
lamps of a constant-current circuit. 

Constant-Potential Generators. 

74. Characteristic Curves of Shunt-Wound Generators. 

— The operation of any dynamo can best be studied by 
inspection of a curve which shows the relation existing be- 
tween the current generated or supplied by the machine 
and the voltage under which it operates. Such curves are 
called characteristic curves, and they are generally plotted 
with current strengths as abscissae and voltages as ordi- 
nates. The characteristic curve of a shunt-wound gener- 
ator is shown as E in Fig. 93, and it is seen therefrom that 
the terminal voltage decreases slightly as the current out- 
put of the machine increases, the speed of the generator 
being maintained constant. 

To obtain the characteristic curve of a shunt-wound gen- 
erator experimentally the machine is run at normal speed, 
and readings are taken of terminal voltage and current out- 
put, the setting of the field rheostat being fixed during the 
test. The setting of this rheostat may be that giving rated 



GENERATORS. 



163 



voltage either at no load or at full load. Fig. 93 indicates 
the latter condition. In some small machines the voltage 
can be reduced to zero without causing excessive sparking 
or extreme temperature elevation, but as a rule the com- 
plete characteristic is obtained only when the field rheostat 
is adjusted for a voltage much below the rated voltage of 
the machine. 



140 


















































120 

gioo 

< 
1- 
-1 

> 80 

< 
z 

1 60 
40 

20 











■ 














E 






VJU 


























\ 


























\ 

\ 
























\ 
I 




































RE! 


ISTANCe 


DROP 








— 



50 75 100 

PERCENT FULL- LOAD CURRENT 



150 



Fig. 93- 



The characteristic curve of a strictly constant-potential 
generator would be a straight horizontal line, since this 
indicates that the voltage remains the same at all loads. 
The terminal voltage of a shunt-wound dynamo at constant 
field excitation and speed decreases slightly as the load in- 
creases, because of the armature resistance drop and arma- 
ture reaction, § 50. The armature resistance drop, being 
the product of the armature current and resistance, is prac- 
tically a linear function of the load (the change in resist- 
ance due to heating occasioned by increased current may 
be neglected), and may be plotted as a straight line, as in 



164 DYNAMO ELECTRIC MACHINERY. 

Fig- 93- The total voltage generated is obtained by add- 
ing the armature resistance drop to the terminal voltage. 
Thus, the curve of total voltage, E t , is plotted by adding 
the ordinates of E and those of the resistance-drop curve. 
The difference between Et and the no-load terminal voltage 
of the machine shows the effect of armature reaction. 

The drop in terminal voltage is at first due chiefly to 
the drop resulting from armature resistance. As the cur- 
rent increases, the effects of armature reaction and satura- 
tion of the magnetic circuit become evident. This soon 
becomes the predominating cause of voltage drop, and to 
such an extent that the curve turns back toward the 
origin. When the resistance in the external circuit is zero, 
of course no current flows through the field, and the few 
volts then produced are due to residual magnetism. Unless 
the field excitation is kept constant in determining the 
terminal voltage curve of a generator, it should be re- 
membered that the difference between Et and the no-load 
terminal voltage is also due to a decrease of the field cur- 
rent occasioned by the fall of potential at the terminals 
of the field winding. 

The voltage of a shunt machine generally increases 
more rapidly than the speed. An increase of speed not 
only increases primarily the number of volts generated, but 
also increases the armature flux because of increased 
excitation. The condition of the magnetic circuit as re- 
gards saturation determines whether this secondary influ- 
ence shall be great or small. 

75. Voltage Regulation. — Shunt -wound generators are 
so designed that the lowering of terminal voltage from no 
load to full load shall be as small as is consistent with 
economy and practicability. Such machines are particu- 



GENERATORS. 165 

larly adapted for constant-potential distribution. The 
maintenance of a perfectly constant terminal voltage is 
effected by the use of regulating field rheostats. 

Suppose the field rheostat of a shunt-wound generator 
to be adjusted for obtaining the rated voltage of the 
machine at full load. Upon disconnecting the load and 
leaving the rheostat setting unaltered, the terminal voltage 
of the generator increases. This change of voltage from 
full load to no load at constant speed when expressed as 
a percentage of the rated full-load voltage, is termed the 
voltage regulation of the generator. Thus the regulation 
of the generator of the foregoing section at full load, as 
obtained from Fig. 93, is 

128 — no 



no 



= 0.163 or 16.3 per cent. 



The regulation of a separately excited generator should 
be determined at constant excitation. The regulation of 
a generator unit, consisting of a generator united with a 
prime mover, should be determined at constant conditions 
of the prime mover ; i.e., constant steam pressure, head, 
etc. It would include the inherent speed variations of the 
prime mover. For this reason the regulation of a genera- 
tor unit is to be distinguished from the regulation of either 
the prime mover or of the generator contained in it, when 
taken separately. 

76. Hand Regulation. — To maintain a perfectly con- 
stant terminal voltage at increased load necessitates an 
increase in the total E.M.F. generated in the machine. 
An inspection of the formula for the electromotive force 
of a generator, 

£ = 2$ S/iO- 8 , 



Ur^ 



+ 



166 DYNAMO ELECTRIC MACHINERY. 

shows that the only quantity that it is practical to vary is 
the magnetic flux through the armature 3> TO . This can 
easily be accomplished by regulating the amount of re- 
sistance in a rheostat, which is in series with the field 

coils, and which therefore 

HAND 

regulator. governs the amount of cur- 

^f^^^^ rent in them > as in Fi S- 94- 

\ colls ^ In distributing current for 

use among a number of con- 
sumers the current is carried 
to feeding-points which are 
near the locality they supply, 
but may be distant from the 
station. It is desirable to 
Flg * 94, keep the pressure at these 

points at a constant value, irrespective of the varying loss of 
potential that is going on because of the resistance of the 
conductors leading to them. To achieve this end feeders 
are employed to carry the current to the feeding-points. 
Each feeder is accompanied by a pilot wire imbedded in 
the insulation. At the feeding-point the pilot wires are 
attached to the feeder terminals, and at the station end are 
attached to a voltmeter, so that the station attendant can 
regulate the pressure not at the machine terminals but at 
the distant distributing point. 

77. Field Rheostats. — For varying the current in the 
shunt field coils of generators, it is usual to employ field 
rheostats which may be mounted on the station switch- 
board together with the usual indicating instruments, or 
on a separate frame. Such rheostats consist essentially 
of high-resistance wire or ribbon with numerous taps con- 
nected to a series of contact studs over which moves a 



GENERATORS. 



167 



contact arm. The resistance units may be in the form 
of cards, bars, bobbins, or grids, according to the capacity 
required. 

A field rheostat, manufactured by the General Electric 
Company, is shown in Fig. 95. It is arranged to be 




Fig. 95. 



placed on the back of switchboards with the regulating 
handle projecting in front. The resistance units are of the 
card form, and are constructed by winding the resistance 
ribbon on tubes of asbestos which are subsequently pressed 
flat. These cards are then assembled, with interposed 
asbestos, in sufficient numbers to make up the required 



168 DYNAMO ELECTRIC MACHINERY. 

resistance of the rheostat. Iron plates, somewhat wider 
than the cards, are introduced at intervals, and thus 
increase the radiating surface. Numerous taps are 
taken from the resistance units to the various contact 
studs. 

Fig. 96 illustrates a rear view of a Westinghouse rheo- 
stat with the bottom plate removed. The resistance unit 




Fig. 96. 

is of the bar type, so called because the resistance wire 
is wound on flat iron bars, but insulated therefrom by a 
layer of fireproof insulating material. By varying the 
size of the wire, resistances may be wound of from .03 
to 400 ohms per linear inch of the bar, with a maxi- 
mum capacity of 4 watts per square inch of surface on 
one side. 

Field rheostats for very large generators consist of re- 
sistance units in the form of iron grids supported in 
frames, which are mounted directly on the floor at some 



GENERATORS. 



169 



convenient point near the switchboard. Fig. 97 shows 
such a rheostat made by the Westinghouse Electric and 
Manufacturing Company. 




Fig. 97. 



Another form of field rheostat, made by the Cutler- 
Hammer Manufacturing Company, is shown in Fig. 98. 
In this rheostat the heat generated is not radiated directly 
from the surface of the wire, but is conducted to a sup- 
porting plate, which then becomes the radiating surface. 
The resistance wires, contacts and lever are mounted on a 
base of insulating material, the whole being carried by an 
iron casing, which prevents the possibility of contact with 



170 DYNAMO ELECTRIC MACHINERY. 

the heat radiating portion of the rheostat. Owing to the 
increased radiating surface thus obtained, a shorter and 
smaller wire can be used for a given volt-ampere capacity 
than if the wire were merely exposed to the air. No con- 
sideration of the mechanical strength of the wire enters 
into the design of this resistance, since it is supported and 
protected by an insulating compound. 




Fig. 98. 



When large generators, such as are used in railroad 
work, have their field-circuits opened, the E.M.F. self- 
induced by the disappearance of the flux in the fields is 
liable to reach such a magnitude as to pierce the insulation 
of the field coils and destroy their usefulness. To obviate 
this, before the field circuit is broken, the field coils are 
connected (Fig. 99) through a high discharge resistance, 
and the current in them is allowed to decay slowly. It is 



GENERATORS. 



171 



thus unattended with any destructive potential differences. 
Arc lights have in several instances been used for this pur- 
pose instead of high resistances. 



Binding} Posts 




Field Switch 




Parallel Resistance 1 

Rheostat, Switch 



Discharge 
Resistance 



Pilot Lamp 0=0= 



Held 



^Inmature 



Fig. 99. 



78. Self-Regulation. — By far the most elegant method 
of constant potential regulation is that in which the main 
current of the machine is utilized in maintaining constant 
the magnetic flux through the armature. This is accom- 
plished by passing all or the greater part of the current 
flowing in the armature a few times around the field 
magnets, so that an increased load on the armature in- 
creases the magnetizing ampere-turns of the field coils. 
These series Herns, when rightly proportioned, can be made 



172 DYNAMO ELECTRIC MACHINERY. 

to compensate for a part, for all, or for even more than all 
of the drop. This device can be used in connection with 
any other form of excitation, as permanent magnets, sep- 
arate excitation, or shunt excitation. In the last case, the 
dynamo is said to be compound wound, as described in 
§ 46. If the machine is designed to maintain a constant 
pressure at some distant feeding-point, instead of at the 
machine terminals, the machine is said to be over-com- 
poimded, since the potential at the terminals will rise on 
increase of load. From 3 to 5 per cent over-compound- 
ing is frequent in machines used to supply lighting circuits, 
and 10 per cent over-compounding is usual in railway 
generators. 

79. Characteristic Curves of Compound-wound Gen- 
erators. — As a compound-wound machine is essentially 
a shunt-wound generator provided with a series field wind- 
ing, the characteristic curve thereof would be the resultant 
of the shunt characteristic and the series characteristic. 
The form of these curves is shown in Fig. 100, the shunt 
characteristic being the same as that for shunt-wound 
dynamos. The voltage induced in the armature by the 
increase of magnetic flux due to the current in the series 
turns is proportional to the current. As the load in- 
creases this voltage will increase, and, if the series wind- 
ing be properly proportioned, the increase of the voltage 
due to the current in the series turns may neutralize the 
decrease of the main voltage occasioned by armature 
resistance and reaction. The form of the curves of Fig. 
100 indicates that such neutralization can occur at only 
one load. If the compensation be complete at full load, 
the machine is said to be flat-compounded. The compound 
characteristic for flat-compounding is shown by the broken 



GENERATORS. 



173 



line, and that for over-compounding is shown by the full 
line in the figure. 



140 
120 






















































===== 














OVER-COMPOUNDED 




— ■=.- 








— -=-- 


=— - 


"~^^ 


100 
80 

60 






















£ 


*^ 




FLAT-COMPOUNDED 


















































20 



















^A0f- 












~ ~ 


rz?Z. 


^ rr: 


^"~ 


"""" 











50 , 75 100 

PERCENT" FULL- LOAD CURRENT 



150 



Fig. 100. 

The degree of compounding may be changed by varying 
either the current flowing through the series winding or 
the number of turns on it. In practice it is usual to pro- 
vide more series turns than required, and to place an ad- 
justable resistance across the terminals of the series field 
coils. The full armature current therefore divides between 
this resistance and the series coils, and the amount flowing 
through the latter may be adjusted for the required com- 
pounding. 

In over-compounded machines, the voltage regulation is 
the ratio of the maximum difference in voltage from a 
straight line connecting the no-load and full-load values of 
terminal voltage as function of the current, to the full-load 
terminal voltage. 

80. Railway and Lighting Generators. — The tendency 
of modern engineering practice is to install lighting gener- 



174 



DYNAMO ELECTRIC MACHINERY. 



ators which are directly connected to the prime mover. 
Owing to the inherent speed of steam engines being smaller 
than that of generators, direct-connected armatures are 
designed to run at a lower speed than belt-driven ones. 
Economical construction demands that they be of the mul- 
tipolar type. They require less floor space per kilowatt than 
the belt-driven machines ; and this is a question of consid- 
erable importance in many installations. They have a 
higher efficiency of operation consequent upon the elimina- 
tion of losses in belting and countershafting. They also 
permit of operation of isolated plants in residences and 
other places where the noise resulting from belt-driven 
machinery would not be tolerated. 

In order that standard generators may be easily con- 
nected with engines of any make, and vice versa, commit- 
tees from engineering societies have recommended the 
adoption of the following standard sizes, speeds, and ar- 
mature shaft fits : — 



Sizes in K.W. Capacity . 
Speeds in Rev. per Min. . 
Armature Fit in Inches . 


5 
3 


7-5 

425 

3 


10 
400 


15 

375 


20 

35o 

4 


25 

325 

4 


5o 
290 

5 


Sizes in K. W. Capacity . 
Speeds in Rev. per Min. . 
Armature Fit in Inches . 


75 

275 

6 


100 

250 
7 


125 
235 
1% 


150 

220 
8 


200 

200 

9 


250 

190 

10 


300 

180 

11 



Fig. 1 01 shows a 1600-K. W., 16-pole, 100 rev. per 
min. General Electric Company direct-connected engine- 
driven railway generator. These generators are built in 
sizes from 100 K. W. to 2700 K. W., and are designed to 
yield the prevailing full-load railway voltages of 550, 575, 
or 600 volts. The field-magnet yoke is of cast iron, circu- 



GENERATORS. 



175 



lar in shape and of oval or rectangular cross-section. The 
frame is divided, the upper half being fastened to the lower 
by concealed bolts. The poles are solid steel castings 




Fig. 101. 



bolted to the frame, and may be removed laterally without 
taking out the armature. Commutating poles are provided 
in most sizes, to compensate for armature reaction, thus in- 



176 DYNAMO ELECTRIC MACHINERY. 

suring good commutation at all loads. The armature spider 
is equipped with vanes which fan air through the ventilat- 
ing passages formed through the laminations and wind- 
ings and around the poles, thus improving ventilation. The 
brush-holder mechanism consists of a ring concentric with 
the axis of the armature and attached to the field frame. 
The simultaneous shifting of the brushes is accomplished 
by the turning of the hand wheel. These generators are 
rated on the basis that after a continuous full-load run of 
24 hours the temperature elevation of no part of the ma- 
chine will rise more than 35 above the engine-room tem- 
perature. A subsequent increase of 50 per cent full load 
for two hours will cause no more than 55 C. temperature 
elevation over the surrounding air. 




Fig. 1:02. 

A belt-driven, 400-K.W., 375 rev. per min. generator 
manufactured by the Western Electric Company is shown in 
Fig. 102. The pole pieces are of laminated sheet steel, and 
are cast into the circular yoke, thus insuring good magnetic 
joints. In the larger machines the frames are divided ver- 



GENERATORS. 



177 



tically, a construction that permits of easy access to the 
armature without necessitating the use of heavy hoisting 
apparatus. Slide rails are provided upon which the ma- 
chines may be moved by means of a screw in order to 
tighten the belt. Alignment is maintained by tongues in the 
base of the machine which fit into grooves in the slide rails. 

The Allis-Chalmers Company manufactures generators 
of the belted "H " type in sizes of from 7.5 to 500 K.W. 
for voltages of 120, 240 and 500 volts, and engine-type 
generators from 12 to 1000 K.W. The field poles of 
these machines are made up of laminated steel stampings 




Fig. 103. 

of the shape shown in Fig. 103. In assembling these 
punchings to form the poles, they are alternately reversed 
with respect to one side. Thus, the face of the pole for a 
short depth contains but one-half as much iron as the main 
body of the pole. This results, under normal excitation, 
in a saturated pole face. It has the same effect in pre- 
venting distortion of the field under the influence of arma- 
ture reaction as saturation of the teeth of the armature 
core. The teeth can therefore be operated at a smaller 
magnetic flux density. The hysteresis losses in the teeth 
can accordingly be made smaller. The thinness of the 
stampings, and the ideally perfect lamination of the pole 
face, permit the use of a smaller ratio of tooth width to slot 
width, without the excessive eddy current loss in the pole 



78 



DYNAMO ELECTRIC MACHINERY. 



face which would occur in other machines. The possibility 
of using narrow teeth results in a reduction of the induc- 
tances of the armature coils. This facilitates effective com- 
mutation. 




Fig. 104. 



Fig. 104 shows a 350K.W. engine-type generator made 
by the Westinghouse Electric and Manufacturing Company. 



GENERATORS. 



179 



Some of the important features of this design are the use 
of conductor retaining wedges in the armature slots, the 
arrangement of the series field coil connections, removable 
pole pieces, and the arrangement of armature equalizer 
rings and of the brush-holder shifting device. 




Fig. 105. 



Fig. 105 depicts a General Electric Company generator 
direct coupled to a steam turbine, and mounted on a com- 
mon bedplate. The cut shows the generating unit as 
semitransparent so as to reveal the interior parts. 

The field-magnet frame of a 6-pole single-coil type of 
Lundell generator with its field coil in place is shown in 
Fig. 106. The frame is divided in a vertical plane which 
is perpendicular to the axis of the armature. 



i8o 



DYNAMO ELECTRIC MACHINERY. 




Fig. 106. 

81. Three-Wire Generators. — The adoption of three- 
wire systems of electrical distribution, particularly for light- 
ing, is due to the saving of copper in the line conductors. 
The standard voltage for incandescent lamps is about no 
volts. At this pressure large conductors must be em- 
ployed on long lines in order to maintain a fairly constant 
voltage at the lamps as the load changes. By doubling the 
voltage across the mains and connecting the lamps thereto 
so that they are joined by pairs in series with each other, 
only one-fourth as much copper need be used, since the 



GENERATORS. l8l 

same power is transmitted at half the current, and for the 
same permissible drop the conductors need be but half as 
large. But, in order that each lamp may be operated in- 
dependently of the others, a balancing wire or neutral 
wire must be provided, and this is usually of the same 
size as the other conductors. Therefore the weight of con- 
ductors on a three-wire system will be \ + \ = § as much 
as on a two-wire system. 

The introduction of a neutral wire involves the genera- 
tion of the total E.M.F. in two parts so that this neutral 
wire may constitute a common conductor for the two com- 
ponent voltages. To obtain this 
condition, two generators may be 
connected as in Fig. 107, but as 
this signifies additional expense, 
various other methods have been 
adopted to obtain three-wire dis- 
tribution. These practical methods Flg ' I07 ' 
employ: (a) dynamotors, § no, which have two armature 
windings upon the same core connected to two separate 
commutators, and connected in the same manner as two 
individual generators; (b) storage batteries, § 113, of suffi- 
cient number of cells connected between the two outside 
wires, the neutral wire connecting with the middle point of 
the battery; (c) balancers, §111, which are two mechani- 
cally coupled dynamos connected across the outside wires, 
one of which, if the system be unbalanced, will run as a 
motor and drive the other as a generator which supplies 
energy to the more heavily loaded side; (d) three-brush 
dynamos, and (e) three-wire generators. 

The total voltage of a generator could be divided into 
two parts by placing a brush midway between the positive 




182 



DYNAMO ELECTRIC MACHINERY. 



and negative brushes ; but, for satisfactory current collec- 
tion, the coil short-circuited by this additional brush must 
lie in a weak magnetic field. Such an arrangement was 
developed by Dettmar, and is shown in Fig. 108. This 

illustrates a four-pole field-magnet 
frame wound as a bipolar machine 
with two adjacent north poles and 
two adjacent south poles. The 
yokes of such machines between 
oppositely named poles must be 
of sufficient cross-section to carry 
the total flux per pole at a reason- 
able flux density. The tendency 
of the armature current is to crowd 
the flux toward the leading poles, thus resulting in a some- 
what greater voltage between the positive terminal and 
the neutral wire than between the latter and the negative 
terminal. 




Fig. 108. 




Fig. 109. 



The three-wire generator designed by Dobrowolsky is well 
adapted for three-wire supply circuits. Points of the arma- 
ture winding at distances from one another equal to twice 
the pole pitch are connected to one slip ring and the inter- 



GENERATORS. 



I8 3 



mediate points are connected to another slip ring. Brushes 
bearing upon these collector rings connect with the ends 
of a coil wound on an iron core, called a reactor, as shown 
in Fig. 109. The middle point of the reactor, D, connects 
to the neutral wire of the system, the outside wires being 




Fig. no. 



connected to the brushes B. The reactor has a low re- 
sistance, but a large inductance. The electromotive force 
across the terminals C is an alternating E.M.F. ; and, be- 
cause of the large inductance of the coil, very little current 



1 84 DYNAMO ELECTRIC MACHINERY. 

flows through it when the loads on the two sides of the 
system are equal. If the system be unbalanced, the cur- 
rent flowing in the neutral wire, since it is direct current, 
will suffer little impedance in passing through the reactor. 

A 150-K.W., 6-pole General Electric Company genera- 
tor, provided with two slip rings for connection to a reactor 
as just described, is shown in Fig. no. These machines 
are usually wound for 250 volts, so that 125 volts can be 
obtained on either side of the three-wire system. They 
may be flat- or over-compounded to compensate for line 
drop. 

Three-wire generators having a single slip ring for con- 
nection to the neutral wire are manufactured by the Burke 
Electric Company. The reactor forms a part of the arma- 
ture and revolves with it, the middle point of the reactor 
being connected to the slip ring. 

Three-wire generators are extensively used in isolated 
plants for electric lighting and light power service. 

82. Homopolar Dynamos. — A type of direct-current 
generator in which the armature conductors move in a uni- 
directional and uniform magnetic field, and therefore have 
induced in them electromotive forces of constant direction 
and magnitude, is known as the homopolar dynamo, some- 
times also as the acyclic or unipolar dynamo. Fig. 1 1 r 
shows a cross-section of a simple machine of this type, with 
one conductor, A, connected to two slip rings B. The 
magnetic field of the generator is set up by the current 
flowing in the field coils C ; the paths of the lines of force 
being represented by the dotted lines. The current is led 
from the machine by means of brushes which slide upon 
the slip rings and are connected to wires projecting through 
apertures in the field-magnet yoke. 



GENERATORS. 



185 



Single-conductor homopolar dynamos are suitable for 
supplying a large current at low voltage, and even then the 
magnetic flux traversing the air-gap must be large and 
the armature must be run at high speed. As there is little 
demand for such low-voltage generators, homopolar machines 




Fig. in. 

for practical purposes must be designed to generate higher 
voltages. This may be accomplished by increasing the 
number of armatures mounted together to form one ma- 
chine, or by employing several conductors insulated from 
one another and connected in series. The end of one con- 
ductor must be joined to the beginning of another, but this 
end connection must not cut the lines of force, otherwise 
the resultant E.M.F. would be zero. Consequently the 
end connections must be stationary, and this means that 
two slip rings must be provided for each conductor. Limi- 
tations to increasing the number of conductors are the 
available space for the slip rings and the increased brush 
friction. 



1 86 



DYNAMO ELECTRIC MACHINERY. 



The electromotive force generated by a multi-conductor 
homopolar dynamo is 



60 



volts, 



where 



and 



N = number of conductors in series, 
<I> = total flux entering armature, 
V = rev. per min. 



The armature of a 300-K.W., 500-volt, 3000 rev. per 
min., turbine-driven homopolar generator is shown in Fig. 
112. It consists of 12 copper conductors mounted on a 




Fig. 112. 



cast-steel core, the ends of the conductors being connected 
to 12 cast-steel slip rings at either end of the armature. 
One copper brush is provided for each ring, and access is 
obtained thereto through apertures in the cast-steel field- 
magnet frame. Compounding is effected by utilizing the 
M.M.F. of the currents in the stationary leads which are 
connected to the brushes, or that of the currents in the 
slip rings from the connection points to the brushes, to aid 
the M.M.F. of the field current. The degree of com- 
pounding may be varied by shifting the brushes. Homo- 
polar generators may be separately- or self-excited. 



GENERATORS. 



I8 7 



As the air-gap of homopolar machines may be very 
small, the field current need not be great in order to set 
up the required magnetic flux 
through the armature. This 
fact indicates a low excitation 
loss. There are practically no 
iron losses in this type of 
generator because of the con- 
stancy of flux density. The 
brush losses, however, are 
large. The efficiency curve 
of the 300-K.W. generator 
is shown in Fig. 113. The Fig. 113. 

voltage regulation is from 6 to 12 per cent. 



90 

> 
z 80 


u. 

fc 70 

1- 
z 

0. 

50 











































































50 75 100 125 

PERCENT FULL LOAD 



Constant-Current Generators. 

83. Characteristic Curves of Series-Wound Generators. 

— Fig. 114 shows the characteristic curves of a 1 5 K.W. 
series-wound generator at a speed of 1000 rev. per min. 
Curve E indicates the terminal voltage of the machine 
when delivering various currents, and is called the external 
characteristic. This curve shows that the E.M.F. of the 
generator at first increases in proportion to the current 
output, but as the load increases the resistance drop of the 
field and armature windings and armature reaction cause 
the curve to bend back. This is also due to the fact that, 
as the magnetic circuit approaches saturation, the magnetic 
flux does not increase proportionally to the increase of field 
current. To obtain the external characteristic of a series- 
wound generator experimentally the machine is run at a 
definite and constant speed and observations of terminal 



i88 



DYNAMO ELECTRIC MACHINERY. 



voltage and current output are made at different loads. At 
constant load the terminal voltage will vary directly with 
the speed. 

The total characteristic of a series-wound generator may 
be obtained from the external characteristic and the resist- 
ance drop of the windings, § 74. Thus in Fig. 114, curve 

















E * 




















^E 






























// 


















L 


7 


















/ 




















/ 














































&0. 


>#&-- 


oP°£- 





























AMPERES 

Fig. 114. 



Ei is plotted by adding the ordinates of the curves of ter- 
minal voltage and resistance drop. The curves of both E 
and Et start above zero because of residual magnetism in 
the cores of the field magnets. 

The total characteristic resembles the magnetization 
citrve of series-wound generators. The latter is a curve 
which shows the terminal voltage of a machine at no load 
for different values of field current. The difference exist- 



GENERATORS. 



189 



ing between these curves is due solely to the demagnetizing 
effect of the armature current on the magnetic circuit. 

84. Power Lines. — Where volts and amperes are used 
as ordinates and abscissae, lines can be drawn connecting 
points of constant product of the two, representing watts 
or power. Fig. 115 shows such lines drawn for one, two, 

















e\ 






/ 










/ 






^ 




' 











20 30 

AMPERES 



Fig. 115. 



and three kilowatts. If E be the external characteristic of 
a dynamo, then the curves make it apparent that the 
machine cannot generate 3 K.W., but that for most values 
under 3 K.W. there will be two loads under which the 
generator can run and yield the same voltage. 

85. Series-Wound Generators. — The advantage of con- 
stant-current distribution for arc lighting lies in the saving 
of conductor material. In this system, as the load increases 
the voltage must increase a corresponding amount, so that 



190 DYNAMO ELECTRIC MACHINERY. 

the current flowing will be unchanged. An ordinary series 
arc lamp, as it is trimmed and adjusted for general use, re- 
quires between 45 and 50 volts to force its rated current 
through it. A generator supplying a circuit of say 2000 
candle-power lamps with n such lamps in the circuit must 
be capable of generating a constant current of 9.8 amperes. 
It must be able to regulate its pressure between the limits 
of 50 and 50 11 volts. This is necessary in order that it may 
operate all the lamps or any part of the whole number at 
proper illumination. 

' The current of an arc-light machine must not exceed nor 
fall below its normal value, no matter how suddenly the 
load is varied ; for the slightest change affects the intensity 
of the light at the lamps. It is obvious that some mechan- 
ical device could be applied to an ordinary shunt-wound 
generator to cause it to give constant current, either by 
changing the position of the brushes or by varying the 
ampere-turns of the field coils. However, any such device 
would be slow of operation, and a sudden short-circuit 
would cause a destructive current to flow before the reg- 
ulator completed its action. It is therefore necessary to 
rely on the armature reactions for regulation, since they 
vary simultaneously with the current. All successful con- 
stant-current machines are constructed on this principle. 
The machine is designed with a magnetic field of great 
intensity, the armature reactions are very great, and thus 
the total flux effective in producing E.M.F. is reduced. 
A slight increase of current in the armature materially in- 
creases the armature reactions. The effective flux is thus 
reduced, and the pressure falls until the current returns to 
its normal value. Thus the machine is completely and in- 
stantly self-regulating. The field coils are series wound on 



GENERATORS. 191 

all arc-light generators, and the cores of the field magnets 
are worked at a very high magnetic density, since the mag- 
nets are then less sensitive to slight changes in the mag- 
netizing force. In commercial machines the densities in 
the field cores are from 17,000 to 18,000 lines per square 
centimeter for wrought iron or steel, and from 9000 to 
1 1,000 lines for cast iron. 

In the armature high magnetic density is also required 
to prevent a sudden rise of voltage when the circuit is 
broken. With no current in the armature, the total mag- 
netomotive force of the field magnets would be effective in 
producing E.M.F., and a destructive rise of pressure would 
result, since the total M.M.F. of the field magnets is much 
greater than the normal effective M.M.F. But a high 
magnetic density in the armature core leaves the latter 
incapable of receiving such an increase of flux, and there- 
fore destructive voltages are avoided. In practice the 
armature core is designed to have a density of from 1 5,000 
to 20,000 lines per square centimeter at its minimum cross- 
section. 

A consideration of the foregoing theory of regulation 
shows that the following conditions should obtain more or 
less completely in a successful constant-current generator : 
(a) since the current is small, there must be a great number 
of armature turns ; (b) the magnetic field of the machine 
must be much distorted ; (c) the path of the lines of force 
of the field coils must be long and of small area, so the 
M.M.F. cannot be readily changed ; (d) the path of the 
lines of force due to armature magnetization must be short 
and of great area, so that the M.M.F. of the armature will 
change with the slightest change of current ; and (e) the 
pole pieces must be worked at a high flux density. 



192 DYNAMO ELECTRIC MACHINERY. 

Evidently extreme difficulty is found in so designing the 
different parts of the machine as to give proper considera- 
tion to each of the conditions and yet produce a machine 
that will regulate for constant current at all loads. This 
leads to the introduction of automatic mechanical devices 
for aiding in the regulation. These devices must not be 
considered as being the sole regulators, for in every case 
they are secondary to the natural self-regulating tendency 
of the armature. In general they regulate for the gradual 
and greater changes of load, while the armature reactions 
take care of the smaller and more sudden fluctuations. 

There are two general systems of regulating arc-light 
dynamos. The first method is to cause the machine to de- 
velop an E.M.F. in excess of that required for the load, and 
then to collect an E.M.F. just sufficient for the load in 
hand. This is done by shifting the brushes from the neu- 
tral plane (§ 52). In a closed-coil armature this causes a 
counter pressure to be developed in those conductors lying 
between the commutating plane and a similar plane in the 
other direction making an equal angle with the neutral 
axis. This reduces the pressure to the desired amount. 
In an open-coil armature the brushes, when in the maxi- 
mum position, connect to the circuit those coils of the 
armature which at that instant have the maximum E.M.F. 
generated in them. By shifting the brushes either way, 
coils can be connected to the circuit which have some 
E.M.F. lower than the total E.M.F. generated in them, and 
the amount of shifting regulates the pressure on the line. 

The second method of arc-light dynamo regulation is to 
vary the magnetizing force in the field magnets just enough 
to put the required pressure on the line. Since the mag- 
netizing force is dependent on the ampere-turns of the field 



GENERATORS. 193 

coils, it can be varied either by cutting out or short-circuit- 
ing some of the turns or by changing the current in them 
by means of a variable resistance which is shunted across 
the field terminals. In practice both these methods have 
been used. 

Whether regulation is effected by changing the position 
of the brushes, or by changing the field excitation, sparking 
will occur at the points of collection of the current if means 
are not provided to avoid it. Sparkless collection could be 
obtained were the magnetic field perfectly uniform all around 
the armature. In general this condition is impracticable, 
since it requires almost the whole armature to be covered 
by the pole faces, and it requires the density in the gap 
beneath them to be uniform. Considerations of magnetic 
leakage and armature reaction render almost impossible the 
satisfying of these conditions. Another and more practical 
method is to employ for current collection at one terminal 
of the machine two brushes connected in parallel. These 
are moved in opposite directions, thus giving the effect of a 
single brush of varying circumferential contact, the center 
of which can always be kept in the neutral plane. This 
device avoids excessive sparking, and is used quite success- 
fully in practice. There is, however, some question as to 
the advisability of resorting to it. 

86. The Brush Machine. — Fig. 116 shows a standard 
160-light Brush arc-light generator, made by the General 
Electric Company. The armature revolves between the 
pole faces of two sets of field magnets. Like poles are op- 
posed to each other. The flux, therefore, takes a path out 
of the opposing pole faces into the armature core, and then 
circumferentially through the core and out into the next 
pair of opposing pole faces. 



194 



DYNAMO ELECTRIC MACHINERY. 



The armature is of the open-coil type and consists of a 
number of coils or bobbins placed on a ring core of greater 
radial depth than breadth, and the pole faces cover the sides 




Fig. 116. 



instead of the circumference. The individual bobbins are 
protected by an insulating box, but are not surrounded by 
any masses of metal. 



This fact, together with the fact 



that the armature is of such shape as to cause great air 



GENERATORS. 



195 



disturbances, insures exceptionally good ventilation of the 
armature. This machine is of relatively slow speed, the 
larger sizes running at only 500 rev. per min. 

At a given instant of time, the different coils on the 
moving armature have E.M.F.'s of widely different magni- 
tudes induced in them. The commutator, Fig. 117, is so 




Fig. 117. 



designed that it connects the coils of highest E.M.F. in 
series with one another to the external circuit, and con- 
nects the coils of medium E.M.F. in multiple with one 
another to the external circuit, while those of smallest 
E.M.F. are cut out entirely from the circuit. 

The bearings are self-lubricated by means of rings. 
Since the poles are on the sides of the armature, side play 
in the bearings must be prevented. The commutator end 
of the shaft is turned with thrust collars which are engaged 
by corresponding annular recesses in the brasses. 

Voltage regulation on these machines is effected by a 
variable resistance in shunt with the field coils ; and as the 
field current is changed the position of the brushes is also 
changed, not to collect current at a lower voltage, as de- 



196 



DYNAMO ELECTRIC MACHINERY. 



scribed in § 85, but to obtain sparkless collection. These 
two operations are performed by a regulator, shown in Fig. 
118, which is attached directly to the frame of the machine. 
The mechanism consists of a rotary oil-pump driven by a 
belt from the armature shaft, a balance valve of the piston 




Fig, 118. 



type, and a rotary piston in a short cylinder, which is 
directly connected to an arm moving over the contacts of 
the field-shunt rheostat. The valve is operated by a lever 
actuated by a controlling electro-magnet which is energized 
by the whole generator current. At normal current the 
valve is centrally placed, and the oil from the pump flows 
around the overlapping ports into the reservoir without 



GENERATORS. 



197 



effect. If the current rises above the normal, the armature 
of the controlling magnet is attracted, the balance valve 
moves up, and oil enters the cylinder, moving the rotary 
piston in a clockwise direction. The shaft of this piston 
moves the arm of the rheostat, cutting out resistance and 
thus lowering the field exciting current. At the same 




Fig. 119. 



time a pinion on the shaft, seen in Fig. 119, actuates a 
rocker arm which moves the brush holders to a position 
such that the collection of current by the brushes will be 
sparkless. When the current returns to its normal value 
the adjusting spring returns the lever and balance valve to 
the central position. If the current falls below normal 
value, these operations are reversed. It is claimed for this 



198 



DYNAMO ELECTRIC MACHINERY. 



regulator that it can bring the current back to normal from 
a complete short-circuit in from 3 J to 4 seconds. The ten- 
sion of the adjusting spring can be regulated from the out- 
side of the dust-proof case by a hard rubber knob. 

87. The Excelsior Arc-Light Generator. — This machine, 
Fig. 120, is a closed-coil ring-armature generator, having 

pole faces that cover both 
the sides and the circum- 
ference of the armature. 
The interesting feature of 
this machine is the method 
of regulation. The proper 
voltage is supplied to the 
line by using both methods 
of control in conjunction ; 
that is, sections of the field 
windings are cut in or out 
of the circuit, and at the 
same time the position of 
the brushes is shifted. The 
proper motion of the field 
regulator arm and of the 
brush holder is obtained by 
means of a small motor 
whose field is " sneaked " from the main magnets of the 
machine. This motor is operated by a device shown 
in Fig. 121. The whole device is inserted in series with 
one of the mains from the generator. The right-hand 
lever is of insulating material, with the contact blocks a 
and b properly placed upon it. The left-hand lever is of 
conducting material, and is capable of being attracted by 
the electro-magnet which is excited by the main current. 




Fig. 120. 



GENERATORS. 



199 



The magnet and spring are so adjusted that when the nor- 
mal current is flowing, both a and b are in contact with the 
left lever, and the current flows in the three shunt paths, 
R f R v and R r There will be no current in the armature 
of the regulating motor, since the potential at brush x is 




From Dynamo 



Fis. 121. 



equal to the potential at brush y. If now the line current 
becomes too strong the magnet attracts the left lever to 
it and the contact at a is broken. Immediately the current 
flowing through b divides at the brush x> part going through 
R 2 and part through the motor armature and R y . The 
motor will then revolve in a given direction, and by simple 



200 DYNAMO ELECTRIC MACHINERY. 

mechanical devices will cut out sections of the field wind- 
ings, and will shift the brushes until the normal current is 
flowing, when contact is again made at a and the control- 
ling motor stops. If the line current drops below normal, 
the spring pulls the lever away from the magnet and the 
contact at b is broken. Part of the current then flows from 
y to x through the motor armature. It therefore revolves 
in a direction opposite to that which it had before. The 
brushes on the dynamo are shifted back again, and more 
sections of field winding are put into circuit. 
• When the current is broken at a or b, there is no serious 
sparking, since there are always two circuits in shunt with 
the break. The whole current of the dynamo does not 
exceed ten amperes ; and the resistances R, R x , and R 2 are 
so proportioned that only a small portion of this current 
flows through a or b. 

In practice the levers and the magnet are mounted on 
the wall or the switchboard, while the regulating motor is 
mounted on the dynamo frame. 

88. The Thomson-Houston Dynamo. — The Thomson- 
Houston arc-light generator is of a type entirely different 
from the other machines here described, not only in appear- 
ance, but also in method of armature winding and of regu- 
lation. A view of this machine is given in Fig. 122. Each 
field coil has for its core an iron tube, flanged exteriorly at 
each end to form a recess for the windings, and fitted at 
the armature end with a concave iron piece that surrounds 
part of the armature. This tube, with the flanges and the 
cup-shaped end, is cast in one piece. The farthermost 
flange of each field core is bolted to a number of wrought- 
iron connecting-rods which hold the magnets in place, pro- 
tect the field windings, and take the place of the yoke of 



GENERATORS. 



201 



other machines in completing the magnetic circuit. The 
magnets are mounted on a frame, including legs and bear- 
ing supports for the armature shaft. 

The armatures of the older machines of this type are 
spheroidal in shape, while the more recent ones have ring 




Fig. 122. 

armatures ; these are more readily repaired or rewound. 
The winding of either form of armature is peculiar in that 
only three coils are employed, set with an angular displace- 
ment from one another of 120 degrees. The inner ends 
of the three coils are joined to each other, and are not at- 
tached to any other conductor, an arrangement unique in 



202 



DYNAMO ELECTRIC MACHINERY. 



direct-current dynamos. The outside ends are connected 
to the segments of a three-bar commutator, from which the 
current is collected by four copper brushes connected in 
multiple. 

Regulation is obtained by shifting the brushes in the 
following manner. Fig. 123 shows the two possible rela- 
tions between brushes and commutator that may exist at 
any instant. Both brushes of each set may rest on one 
commutator bar, or the brushes of one set may span the 
gap between the other two bars. These conditions are re- 
peated three times at each brush for each revolution. If 




Fig. 123. 



the dotted line shows the position where the maximum 
E.M.F. is generated in the coils, then in Fig. 123 a the two 
most active coils are connected in series with the outside 
circuit, while the coil near the position of least activity is 
out of circuit. In Fig. 123 b the two less active coils are 
in multiple with themselves and in series with the most 
active coil and the external circuit. In practice the brushes 
of a set are 60 degrees apart, leaving 1 20 degrees between 
the leading brush of one set and the following brush of the 
other set ; and since 1 20 degrees is the angular measure 



GENERATORS. 



203 



of the length of a commutator bar, there is no coil out of 
circuit at normal load, two being always in parallel and in 
series with the third. If the current rise above the normal 
the leading brushes move a small angle forward, while the 
following brushes recede through three times that angle. 
This will shorten the time that a single coil gives its whole 
E.M.F. to the circuit, and will place it more quickly in par- 
allel with a comparatively inactive coil. But such a move- 
ment will reduce the angular distance between the nearest 
brushes of the opposite sets to less than 120 degrees, hence 
the machine will be short-circuited six times per revolution, 
since one brush of each set will touch one segment of the 
commutator at the same time. If the current in the line 
falls below normal, then the brushes close together, and 
the time that a coil is in series is lengthened, and the time 
that it is in parallel with an inactive one is lessened. 




The arrangement for moving the brushes is shown in 
Fig. 124. The leading brushes are shifted forward on an 
increase of current merely to help avoid sparking. The 



204 DYNAMO ELECTRIC MACHINERY. 

brushes are moved by levers actuated by a series magnet 
A. This magnet is normally short-circuited by the by- 
pass circuit. On an undue rise of current this circuit is 
broken by the series magnet B. A then becomes more 
powerful, and the levers separate the brushes. While the 
machine is in operation the circuit-breaker C is constantly 
vibrating, the brushes adjusting to suit the load. A high 
carbon resistance is shunted across C to prevent sparking 
at that point. 

As might be expected, with but three parts to the com- 
mutator and collection made with small regard to the 
neutral point, the sparking of this machine is such as speed- 
ily to ruin the commutator and the brushes, if means 
are not taken to suppress it. A rotary blower is mounted 
on the shaft, and is arranged to give intermittent puffs of 
air, which at the right moment blow out the spark. The 
insulation between the segments is air, considerable gap 
being left between them, and through these gaps the sparks 
are blown. 

89. Western Electric Arc -Light Dynamo. — Fig. 125 
represents a Western Electric Company generator, which 
is regulated by means of shifting the brushes. The brush 
and rocker are connected by means of a link and a ball-and- 
socket joint with a long screw, the latter being held in posi- 
tion by a nut. When the current is normal, both the nut 
and screw revolve at the same rate, and consequently there 
is no axial movement of the screw, and the brush, there- 
fore, remains stationary. An electro-magnet, energized by 
a coil which is in series with the main circuit, attracts an 
armature whose movement toward the magnet is opposed 
by the action of a spring which is susceptible of adjust- 
ment. When the current has too high a value, the electro- 



GENERATORS. 



205 



magnet attracts its armature more strongly than ordinarily. 
The latter moves toward the magnet, and by its movement 
catches a stop on the revolving nut, and thereby prevents 
the revolution of the nut until the resulting longitudinal 
movement of the screw has shifted the brushes sufficiently 




Fig. 125. 



to bring the current to its normal value. If the current be 
too weak, the spring which is attached to the electro-magnet 
armature verpowers the magnetic attraction. The result- 
ing movement of the armature stops the rotation of the 
screw and permits the rotation of the nut. This results in 
a longitudinal movement of the screw and a shifting of the 



206 



DYNAMO ELECTRIC MACHINERY. 




Fig. 126. 



PROBLEMS. 207 

brushes in the opposite direction. The stopping and start- 
ing of the nut and screw are accomplished through the 
medium of small triggers controlled by the armature of 
the series magnet. The triggers are fastened to the gear 
rotated from the main shaft by a belt, and they engage with 
stops on the nut and screw respectively. Fig. 126 gives 
a sectional view of the regulator. The trigger which 
engages with the screw is shown at n^ and the one which 
engages with the nut is shown at m. 



PROBLEMS. 

1. The resistance of the field winding of a generator which 
has been standing idle for a considerable time in an engine-room 
at a temperature of 25 C, is 22.1 ohms. The resistance of 
this winding when the generator is in operation at full load for 
several hours is 25 ohms. Determine the temperature elevation 
of the field coils. 

2. Estimate the output of a generator the armature core of 
which is 18 inches in diameter and 13 inches in length ; the 
speed of the machine being 500 rev. per min. 

3. From the following data of a 350-K.W., 250-volt, 90 rev. 
per min., 16-pole, shunt- wound generator, determine the arma- 
ture core losses: 

Armature diameter 108 inches 

Gross length of armature 15 inches 

Net length of armature 12 inches 

Number of slots (open type) 576 

Depth of slot 1.0 inch 

Width of slot 0.3 inch 

Radial core depth back of slots 7 inches 

Conductors per slot 2 

Size of armature conductors 0.3 X 0.2 inch 

Flux per pole at full load 16 megamaxwells 



208 DYNAMO ELECTRIC MACHINERY. 

Type of armature winding simplex ; lap 

Peripheral length of pole face 15 inches 

Radial length of air-gap 0.3 inch 

Number of field turns per pole 600 

Mean length of a field turn 59 inches 

Cross-section of field conductor 0.03 sq. in. 
Drop at the carbon brushes 2.4 volts 

Current density at brushes 40 amperes per sq. in. 

Diameter of commutator 72 inches 

Axial length of commutator 9 inches 

Exposed surface per field coil I2 5° sq. in. 

- Field pole shoes are of laminated steel. 

4. What is the armature copper loss at full load of the gener- 
ator of the foregoing problem ? 

5. Compute the pole-face loss of the 350-K.W. generator, 
the data of which are given under Prob. 3. 

6. Calculate the excitation loss of the generator of Prob. 3 at 
full load. 

7. Find the total commutator losses at full load of the 
350-K.W. generator of Prob. 3. 

8. Determine the temperature elevations of the armature, 
field coils, and commutator of the generator discussed in the 
foregoing problems, when operating continuously at full load. 

9. A motor-generator set consists of a direct-current generator 
coupled to an alternating-current synchronous motor [power-fac- 
tor = 1]. When the generator delivers a current of 600 amperes 
with 250 volts across the machine terminals, the motor takes 
26.4 amperes at 6600 volts. Determine the efficiency of the 
motor-generator set. 

10. The E.M.F. of a shunt-wound railway generator rises 
from 550 volts at full load to 645 volts upon disconnecting the 
load. What is the percentage regulation of the machine ? 

11. A shunt-wound generator, rated at 50 K.W., supplies 
current to an external circuit with 550 volts across the machine 



PROBLEMS. 209 

terminals. To produce this voltage 6280 ampere-turns per pole 
are required at no load, and 7640 ampere-turns at full load. 
How many series field turns per pole must be provided for flat 
compounding ? 

12. A three-wire system supplies current to no- volt lamps 
and to a 220-volt motor which is connected to the outside wires. 
It is found that the lamps on one side of the system burn more 
brightly than those on the other, while the motor operates as 
usual. What is the trouble ? 

13. Find the flux density in the air-gap of the 300-K.W., 
500-volt homopolar generator mentioned in § 82 ; the armature 
diameter being taken as 20 inches and its net axial length as 
12 inches. 

14. When a series-wound generator is driven at 1200 rev. 
per min. its terminal voltage is 150 volts, with a current output 
of 20 amperes. Compute the terminal voltage of the machine 
when it delivers a current of 50 amperes at a speed of 1500 
rev. per min. ; the resistance of armature and field windings 
together being 0.125 ohm. The increase in magnetic flux 
accompanying the increased current is 60 %. 



2IO DYNAMO ELECTRIC MACHINERY. 



CHAPTER VII. 

MOTORS. 

go. Principle of Action of a Motor. — Any direct-current 
generator will operate as a motor and deliver mechanical 
energy if supplied with current from some external source. 
This source may be a constant-potential system or a con- 
stant-current system of electrical distribution. Structur- 
ally generators and motors are identical, but as motors are 
generally placed as near as possible to their loads, they may 
frequently be exposed to severe weather conditions, dirt, 
etc., and for this reason motors for electric railways, for 
rolling mills and for machine tools are of the enclosed 
type. 

When a current flows through a conductor which is situ- 
ated in a magnetic field, a force will be exerted upon that 
conductor tending to move it perpendicularly to itself and 
to the magnetic flux ; the magnitude of this force in dynes 

being F = — -, (§ 22) 

where I = current flowing in conductor in amperes, 

/ = length of conductor in centimeters, 
and (B = flux density of magnetic field in gausses. 

Irrespective of the multipolarity of the field magnets or of 
the method of armature winding, the force actions between 
the magnetic field and all the currents in the inductors will 
conspire to produce rotation in one direction. 



MOTORS. 



211 




Consider a single armature conductor, Fig. 127, to carry 
a current flowing away from the observer. The lines of 
force which surround the con- 
ductor due to the current in it 
will have a clockwise direction. 
Thus, to the right of the con- 
ductor these lines will have the 
same direction as the lines of 
force from the field magnet A 7 ", 
and to the left of the conductor 
they will be opposed to the latter. 
The resultant field, therefore, will be stronger on the right- 
hand side, as shown ; and consequently the armature carrying 
that conductor will be pushed to the left and will rotate 
counter-clockwise. 

91. Direction of Rotation. — To determine the direction 
of movement of a conductor carrying a current of definite 
direction in a magnetic field of known direction, one may 



Fig. 127. 



'Kern* 




FIELD MAGNET 

DYNAMO • RIGHT HAND. 

Fig. 128. 



CURRE* 1 




MOTOR LEFT HAND. 

Fig. 129. 



employ a modification of Fleming's rule. Thus in a gen- 
erator the thumb and first two fingers of the rigJit hand 
determine the direction of the induced E.M.F. as shown 
in Fig. 128. But in a motor the thumb and first two 



212 



DYNAMO ELECTRIC MACHINERY. 



fingers of the left hand may determine the direction of 
rotation as shown in Fig. 129. 

If in a dynamo the direction of the field flux remain 
unaltered, and the armature be supplied with a current 
flowing in the same direction as when the machine was 
operated as a generator, then the direction of rotation will 
be opposite to that while driven as a generator. Thus, if 
the positive brush of the generator be connected to the 
positive terminal of an external source of supply, and if the 
negative brush be connected to the negative terminal, then 
the direction of current flow in the armature will be 
reversed. The connections of shunt-wound and series- 




Fig. 130. 



Fig. 131. 



wound dynamos are shown respectively in Figs. 130 and 
131, in which the full arrows represent generator condi- 
tions and the dotted arrows represent motor conditions, the 
connections to the circuit remaining unchanged. In shunt- 
wound, separately excited and magneto machines, since the 
magnetic fields in these dynamos are not reversed, the 
direction of rotation will be unaltered. The direction of 
rotation of the armature of series-wound dynamos, since 
the field flux also has its direction changed, will be re- 
versed. Compound-wound machines will have the same or 
reversed direction of rotation, depending upon whether the 
magnetizing effect of the shunt coils is stronger or weaker 



MOTORS. 213 

than that of the series coils. In a compound-wound gen- 
erator the actions of the shunt coils and the series coils are 
cumulative, i.e., in the same direction ; but when used as a 
motor the actions are differential, i.e., opposed to each other. 
Motors are also wound so as to have cumulatively acting 
series coils. 

To reverse the direction of rotation of a motor one must 
not change the connections with the supply mains, for this 
would reverse the current directions in both armature and 
field windings, and thus leave the direction of rotation unal- 
tered. It is necessary to change the connections of either 
field or armature winding, but not of both. 

92. Torque Exerted by a Motor. — The force which is 

exerted upon each conductor carrying a current / and 

situated in a uniform magnetic field of flux density (E is 

7/(B 

dynes, § 90. The total number of conductors on the 

armature which are under the 2p poles may be represented 

as kSq, 

where k is the ratio of the circumferential length of the 
pole face to the pole pitch, 5 is the number of conductors 
in series between brushes, and q is the number of current 
paths through the armature between brushes. Let I t be 
the total or external armature current in amperes, and D 
be the diameter of the armature in centimeters. Then the 
total torque exerted by the armature in dyne-cm. is 

T = - - ^ • /<B • ^ = 0.05 kDl&SIt. 

2 q IO 

But the total flux per pole is 

knDl(S> 
$ = -. 

2/ 



214 DYNAMO ELECTRIC MACHINERY. 

Therefore the total torque in dyne-cm. is 

T= - .05 pSQIt = -^- 54)/,, 

- IO- 

which shows that the torque exerted by the motor is pro- 
portional to the magnetic flux and to the armature current. 
Since there are 980 X 453-6 X 30.48 or 13,549,000 dyne- 
centimeters in one pound-foot, the torque in pound-feet may 
be expressed as 

T = 2.35 pSQIt 10- 9 . 

•The effective torque available at the pulley of the motor 
is somewhat less than that given by the foregoing equation, 
due to the mechanical and iron losses. 

When load is placed upon a motor, such as machinery in 
one form or another, a certain torque must be exerted which 
is equal to the torque-reaction of the load. With greater 
load more torque must be exerted, and therefore the prod- 
uct 4>/ t must become larger. As a result a motor takes 
more current when operating under heavy load than when 
running light. 

93. Counter Electromotive Force. — The armature of a 
motor revolving in a magnetic field under the influence of 
supplied electrical energy differs in no respect from the 
same armature revolving in a magnetic field under the in- 
fluence of supplied mechanical energy. There is an E.M.F. 
generated in it which is determined by the speed and quan- 
tity of flux. For the same speed and the same flux there 
would be generated the same E.M.F. in the case of a motor 
as in the case of a generator. The direction of this E.M.F. 
is, however, such as to tend to send a current in a direction 
opposite to that of the current flowing under the influence 
of the external supply of E.M.F., according to § 91. 



MOTORS. 215 

Therefore this pressure which is induced in the armature 
of a motor is called counter electromotive force. The cur- 
rent which will flow through the inductors of an armature 
is therefore equal to the difference between the supply 
E.M.F. and the counter E.M.F. divided by the resistance 
of the armature, or 

T _ & ^ G 

a ~ Ra ' 

For example, an unloaded i-K.W. shunt-wound motor 
having an armature resistance of 1 ohm, when connected 
to a source of constant -potential supply of 100 volts, would 
not take a current of 100 amperes as dictated by Ohm's 
law, unless its armature were clamped so as to prevent ro- 
tation. If undamped, its armature would assume such a 
speed that it would have induced in it a counter E.M.F. of 
say 97.5 volts. The current then flowing in the armature 
would be 

100 — 97.5 

=2.5 amperes. 

The power represented by this current, viz., 2.5 x 100 
watts, would all be expended in overcoming the losses of 
the machine. 

The magnitude of this counter E.M.F. in volts is 

£c=2/"i>5^io- 8 ) (§39) 

where 3> is the flux per pole in maxwells, and V is the 
speed in rev. per min. 

If the load upon a motor be increased, its torque is no 
longer sufficient to overcome the load and consequently its 
speed drops. A lowering of speed implies the generation 
of a lower counter E.M.F., and thus permits a greater 



216 



DYNAMO ELECTRIC MACHINERY. 



current to flow through the armature. The greater cur- 
rent results in a greater torque. 

94. Armature Reactions. — Since in a motor, for a given 
direction of rotation and of flux, the current in the arma- 
ture flows in a direction contrary to that which it would 

have as a generator, therefore the 
effect of the motor armature cross 
turns is to distort the magnetic field 
against the direction of rotation, as 
in Fig. 132. This increases the flux 
density in the leading pole tips, and 
decreases it in the trailing tips. This 
necessitates, for sparkless operation, 
a backward lead, or a lag, of the 
brushes. If the brushes were in the 
same place as when the machine was 
operated as a generator, the direction 
of armature current having been re- 
versed, then the demagnetizing or 
back turns of the generator would 
become magnetizing turns for the 
motor ; but with the brushes shifted to a position of lag, 
then the motor has also demagnetizing or back turns. 

95. Power of Motors. — The mechanical power of a 
motor when running at V rev. per min. and exerting a 
torque T dyne-centimeters is equal to the product of the 
angular velocity in radians per second into the torque 




Fig. 132. 



P 



60 



But the torque exerted, in dyne-centimeters, is 
P 



T = 



10 



SQIt. (§ 92) 



MOTORS. 217 



Therefore P 



2 ^ S 6o\To 



But the quantity in the brackets is equal to 10 s E c . (§93) 
Whence 



dyne-cm. _ T 

— = EJt watts. 



P = E c I t io 7 

sec. 

Thus, the rate at which a motor does mechanical work is 
equal to the product of the counter electromotive force gen- 
erated, in volts, into the total current flowing through the 
armature in amperes. 

Shunt Motors. 

96. Speed of Shunt Motors. — In shunt -wound motors 
connected to constant-potential supply circuits the field 
current is constant and consequently the magnetic field is 
of unvarying intensity. Solving the equations of § 93 for 
speed, there results 

(E -I a R a )6o. io« 

2p$S 

and therefore if <£ is constant the speed of the motor will 
be practically constant. It will not be absolutely constant 
because the small resistance drop occasions a slight lower- 
ing of speed with increased load on the machine. On the 
other hand the effect of the armature current is to weaken 
the magnetic field, if the brushes be displaced backward 
from the neutral plane, and thereby tend to increase the 
speed. This partially counteracts the lowering of speed 
due to resistance drop. The speed variation of shunt 
motors from no load to full load ranges from 2 to 10 per 
cent of the speed at no load, the lower value representing 
that for large machines. 



218 DYNAMO ELECTRIC MACHINERY. 

An inspection of the foregoing equation suggests the fol- 
lowing possible ways of controlling the speed of a motor: 
(i) changing the exciting current in order to change the 
magnetic flux passing through the armature, (2) changing 
the resistance of the armature circuit, and (3) changing 
the impressed electromotive force. A slight change of 
speed can be effected by shifting the position of the 
brushes, for at a given load the speed is a minimum with 
the brushes in the neutral plane, and it will be increased 
by a lag of the brushes ; commutation difficulties limit the 
speed variation by this method. 

(1) A rheostat placed in the field circuit of a shunt 
motor may be used to vary its speed, Fig. 133. By 

increasing the amount of 




mmw 



Fig. 133. 



resistance in this rheo- 
stat the current in the 
field coils will be de- 
creased; this results in 
a weaker magnetic field, 
and consequently the 
shuntHeid motor will run at a 
higher speed. If the 
iron of the magnetic cir- 
cuit is well saturated, a considerable change in resistance 
is necessary materially to alter the field intensity. A large 
exciting current is then required to increase the magnetic 
flux, and this may occasion excessive heating of the field 
coils. Again, if an attempt be made to reduce the flux to 
a considerable extent by introducing more resistance to 
obtain a high speed, the demagnetizing effect of the arma- 
ture current will be greater upon the weakened magnetic 
field, and consequently serious sparking will result. Thus 



MOTORS. 



219 



armature reaction limits speed variation. A shunt motor 
of the usual type which operates at a speed of V rev. per 
min. when the iron of its magnetic circuit is near saturation 
will operate satisfactorily at any speed up to say 2 V revolu- 
tions per minute. Field rheostats are described in § 77. 

In order to vary the speed of a shunt motor over a wide 
range by this method it is necessary to neutralize the effect 
of armature reaction. This neutralization is accomplished 
by the provision of a reversing magnetic field obtained by 
the insertion of auxiliary poles, called commutating-fioles 
or inter-poles y between the field-magnet poles. The coils 
on these auxiliary poles are connected in series with the 
armature, as shown in Fig. 134, and therefore the magnetic 



i 

1 

I 




Fig. 134. 



flux from them is practically proportional to the armature 
current. The reactance voltage (§ 57) generated in a short- 
circuited armature coil due to its rotation in the main mag- 
netic field is also proportional to the current flowing in it. 
The M.M.F. of the interpoles is adjusted so that a magnetic 
field is produced in the commutating zone of such magnitude 
that an E.M.F. is generated in the short-circuited coils by 
their rotation which is equal but opposite to the reactance 
voltage. The action of the interpoles is therefore entirely 



220 DYNAMO ELECTRIC MACHINERY. 

automatic and enables sparkless commutation at all loads 
and speeds. Interpole motors are particularly adapted for 
individual motor drive of machine tools and for elevator 
operation, where large speed variations are essential. The 
Electro-Dynamic Company manufactures such motors, Fig. 




Fig. 135. 

135, which operate at a speed of from 100% to 600% of 
the minimum speed. 

The data of a 5-H.P. interpole motor follow : 
Resistance of shunt field 175 ohms, 

Resistance of armature 1.18 ohms, 

Resistance of interpolar windings 0.2 1 ohm, 
Armature current at full load 22 to 24 amperes, 
Field current 0.15 to 1.26 amperes, 

Speed 200 to 1 200 rev. per min., 

Weight 1200 pounds. 



MOTORS. 221 

A change in magnetic flux can also be accomplished by 
varying the reluctance of the magnetic circuit, the field 
current remaining unaltered. The reluctance may be in- 
creased by lengthening the air-gap of the motor ; this 
decreases the flux and consequently produces a higher 
speed. A variable-speed motor depending upon change 
of reluctance for speed control is shown in Fig. 136, 




Fig. 136. 

which depicts a 4-pole machine of the Stow Manufac- 
turing Company. The field cores are hollow and are 
provided with movable iron poles, the positions of which 
are simultaneously shifted by means of hand wheel and 
gears. Large air-gaps are conducive to sparkless com- 
mutation. 

(2) The speed of a shunt-wound motor with constant 
excitation may be varied by introducing a variable resistance 



222 DYNAMO ELECTRIC MACHINERY. 

in the armature circuit. The use of this method of speed 
control is not to be advised save for experimental purposes, 
since it is very wasteful of energy. The PR loss in the 
regulating resistance at certain speeds is considerably more 
than the power required by the motor. Further, the speed, 
when reduced in this way, changes very considerably when 
the load on the motor is altered. 

(3) Changing the electromotive force impressed upon 
the armature of a shunt motor will cause a corresponding 
change in speed. Speed control by this method may be 
accomplished by subdividing the generator voltage into 
two or more components, and by supplying current at these 
different voltages over a number of line wires to the motor. 
A controller is provided by means of which the motor 
armature may be connected to any pair of supply mains, 
the field winding of the machine being always connected 
to a definite pair of them. The main generator voltage 
is subdivided by a set of generators, called a balancer ; 
Fig. 181 illustrates a three-element balancer for a 4-wire 
multivoltage distribution system. The connections of a 
motor to such a system through a controller are shown in 
Fig. 137. Six different voltages are obtainable, namely, 
40, 80, 120, 160, 200 and 240 volts, by moving the con- 
troller handle. The motor speeds under these voltages are 
approximately proportional to the voltages themselves, so 
that this method of speed control gives a number of defi- 
nite and widely different speeds. Intermediate speeds may 
be obtained by weakening the magnetic fields of the motors 
using field rheostats as described. Controllers designed 
to perform both of these functions are also employed. This 
system is extensively used in machine shops for driving 
lathes, planers, and similar machines. 



MOTORS. 



223 



Speed control of shunt motors by varying the voltage 
impressed upon the armature is also the principle of the 
Ward Leonard system, which differs from the foregoing 
multivoltage system in that a finely graded variation of 
speed is made possible by the employment of a separate 
generator which supplies current to the motor. This gen- 
erator, c7, Fig. 138, is driven at constant speed by any type 





m\ 



Fig. 137. 



of prime mover, S, or by an auxiliary shunt motor which 
takes current from the supply mains. An adjustable resist- 
ance in the generator field circuit regulates the voltage 
which is impressed upon the motor armature, M, from prac- 
tically zero to its maximum value. The field winding of 
the motor is connected directly to the supply circuit, so 
that the intensity of its magnetic field is constant. 

When it is desired to start the motor, the rheostat is ad- 
justed so that a high resistance is in circuit with the field 



224 



DYNAMO ELECTRIC MACHINERY. 



winding of the generator; thus current at low voltage will 
be supplied to the motor. The latter then starts to revolve 
slowly. To accelerate the motor, more resistance is cut 
out of the generator field circuit and consequently the volt- 
age across the motor armature terminals increases, thus 
resulting in higher speed. 



^UJy^~ 



« 



JQ 



Fig. 138. 



This system of motor control is especially advocated for 
operating guns and turrets on battleships, where thorough 
control is essential. For the latter, the field rheostats are 
designed to yield seventy or more speeds, the maximum 
speed being usually 100 degrees per minute. The turret- 
turning motors are rated at 25 and 15 H.P. respectively 
for 12-inch and 8-inch turrets. The gun-elevating motors 
are rated at 8 and 5 H.P. for 12-inch and 8-inch guns 
respectively. 

Of the various methods of speed control just described, 
the field rheostat method is perhaps the most used. It is 
simple, cheap, and enables the speed to be kept at definite 
value under changes of load. Its range is limited in shunt 
motors of the usual type, but the interpole motor removes 



MOTORS. 



225 



this difficulty. The change of reluctance method does not 
require a field rheostat, but this is offset by the increased 
cost of construction of such motors. Both the multivolt- 
age and Ward Leonard systems are very practical but ex- 
pensive, since the former requires a balancer, a number 
of live wires, and individual controllers ; and the latter sys- 
tem requires a motor-generator set and rheostat for each 
motor. 

97. Starting of Shunt Motors. When the armature of 
a motor is at rest there is no counter E.M.F. ; and at the 
instant of closing the circuit a destructive current would 
flow if a resistance were not first inserted in the circuit, 
except in the case of very small motors whose armatures 
have small moments of inertia. As the speed rises the 
counter electromotive force in- 
creases and the current is reduced, 
thus permitting the resistance to 
be gradually lessened without caus- 
ing an excessive current to flow 
through the armature. When the 
speed approaches its ultimate value 
this resistance is entirely cut out /f~ 
of circuit. In order that the counter s««>/7 
E.M.F. may be generated the shunt 
field circuit must be closed, so that 
the armature conductors cut lines 
of force. An arrangement for con- 
veniently performing these func- 
tions is called a starting- box or 
starting rheostat. Fig - I39 - 

The connections of a simple starting rheostat are shown 
in Fig. 139. Its main feature is a contact arm capable of 




226 



DYNAMO ELECTRIC MACHINERY. 



rotation about its center so that one end moves over a 
series of contact studs while the other makes contact with 
the segment which is connected to the field winding. As 
the arm is slowly turned around, it first completes the field 
circuit, then the other end touches the first contact stud, 
thereby closing the armature circuit through all the resist- 
ance of the rheostat. As the speed increases the revolv- 
ing arm cuts out more and more of the resistance, until 
finally the armature is operating on the full voltage of the 
supply circuit. 

M 6l 



+- 



►>:* : -£ 



o» 



(2) 



^ 



±=a:. 



From Supply 
Circuit 



oT 



U 



~u 



Release Magnet 



Field 



Fig. 140. 



A shunt motor may have its armature coils destroyed 
by an excessive rush of current resulting from a dropping 
or interruption of the supply voltage followed by a sudden 
renewal after the speed of the armature has fallen. These 
conditions may arise through accidents to supply mains or 
because of an extremely heavy load on mains of insufrl- 



MOTORS. 



227 



cient cross-section. An armature may also be burned out 
by an excessive current due to overloading the motor. The 
resulting lowering of its speed is accompanied by a corre- 
sponding lowering of the counter E.M.F. Again, an abnor- 
mal voltage, which might result from some cross or other 
accident, might cause a destructive rush of current. To 
meet these conditions, starting rheostats are often pro- 
vided with attachments for opening the circuit on no 
voltage or low voltage, 
and others with attach- 
ments for opening the 
circuit on overload. 
Some have both attach- 
ments, but it is modern 
practice to place the 
overload device upon 
the switchboard rather 
than on the starting 
rheostat. Fig. 140 is 
a wiring diagram of a 
shunt-motor starting 
box with automatic re- 
lease and no-voltage 
attachment. 

A view of a motor- 
starting panel with both 
no-voltage and overload 
attachments is given in 
Fig. 141. When the 
handle is placed in the " on " position, the magnet in the 
field circuit holds it there, although a spring tends to 
throw it back. If now, because of low voltage, the cur- 




Fig. 141. 



228 



DYNAMO ELECTRIC MACHINERY. 



rent in field winding and magnet becomes low, the magnet 
is no longer able to retain the handle, and the spring throws 
it to the " off " position, where it stays until the motor is 
again turned on by an attendant. The overload coil is 
connected in series with the motor armature, and on over- 
load becomes strong enough to attract an iron piece. This 
operation places a short-circuit on the release magnet, 
which therefore permits the starting arm to spring back to 
the " off " position. This panel is provided with a main 
switch and enclosed fuses, although the latter are 

frequently replaced by 
circuit breakers. Self- 
contained motor-starting 
panels avoid consider- 
able external wiring, 
thereby increasing relia- 
bility, are quickly in- 
stalled, and add to the 
neatness of the equip- 
ment. 

A combined starting 
and field-regulating rhe- 
ostat made by the Cutler- 
Hammer Manufacturing 
Company is shown in 
Fig. 142. This type of 
apparatus is designed for 
2 to 1 up to 5 to 1 speed 
variation. The movable 
arm consists of two parts 
which wipe over separate sets of contacts. To start the 
motor the handle is moved to the extreme right, in which 




Fig. 142. 



MOTORS. 



229 



position the magnet will hold the lower portion of the 
arm. The upper arm is then free to move back and 




Fig. 143. 

make contact with the studs joined to the field-regulating 
resistance. 

Fig. 143 depicts a General Electric 
Company controller for 5-H.P. shop-tool 
motors. There are three starting points, 
2 1 forward and 1 1 reverse running points. 
Speed control is effected by field regu- 
lation. 

A self-starter for shunt motors made 
by the Ward Leonard Electric Company 
is shown in Fig. 144. It consists of 
an electromagnet with a movable core 
carrying wipers which make contact with 
a series of studs as the core is attracted Flg * I44 * 

by the magnet. The rapidity with which this operation 




230 



DYNAMO ELECTRIC MACHINERY. 



may be performed is controlled by a dash-pot. Thus the 
starting and stopping of the motor is accomplished simply 
and effectively by means of a main line switch. 

98. Design of Starting Rheostats. — The design of a 
starting-box for a shunt motor under constant excitation is 
governed by the permissible starting current through the 
motor armature. This maximum current value is usually 
specified in terms of the full-load current of the machine. 
Let y be the ratio of maximum starting current under load 
to the full-load current ; this ratio is always greater than 
unity. Let r t , r 2 , r 3 , . . . , r be the resistances respectively 
of the starting-box when the rheostat arm is on contact 




Fig. 145. 



studs 1, 2, 3, 



n, Fig. 145. At the instant when the 



arm touches stud 1, the current flowing through the arma- 
ture is 

E 



ri 



r, + R, 



(0 



where E is the line voltage and R a is the resistance of the 
motor armature. When the motor runs at constant speed 



MOTORS. 231 

with this rheostat setting, the current flowing through the 
armature is 

7 = ^f> (2) 

where E CI is the counter E.M.F. generated at this speed. 
The rheostat arm may then be turned to contact stud 2, 
and this results in a momentary increase of current, 

This causes the motor to exert a greater torque than that 
necessary to overcome the load and consequently the motor 
is accelerated and will assume some higher speed. The 
current' will then diminish to 

7 =frf' < 4) 

where E C2 is the E.M.F. generated at the increased speed. 
Similarly, when the arm makes contact with stud 3, the 
current flowing through the motor armature will again 
increase to 

rI = ^Y a - (5) 

From equations (2) and (3) and equations (4) and (5) 
there result respectively 

r_ L± R 3 r_ L± R^ 

r r 2 +R a r r 3 + R a W 

There are n such equations, the last one being 

r n + Rg , 

The number of steps into which the total resistance r^ is to 
be divided, so that the starting current shall not exceed the 



232 DYNAMO ELECTRIC MACHINERY. 

specified value of yl amperes, may be determined from the 
product of these n equations, which is 



r n_ r i+Rg 

7 Rr 



^a 



But from (i) 
consequently 



r, = y T -R a \ (7) 



log 



ylRt 



log r 

If the motor is to be accelerated from rest to full speed 
without any load on it, fewer steps are required, because 
the no-load current is much less than the full-load current. 

The resistance of each of the various steps may then be 
computed ; thus for the first portion between studs I and 
2 the resistance is, from (6), 

r 1 -r 2 = {R a + r 1 )(i- -V 

and similarly the resistance of the next part is 

r 2 -r 3 = (R a +r 2 ) ^i - - j , etc. 

The resistances of the various steps of a starting rheostat 
will be found in examples to differ from one another. Some- 
times additional steps are provided, so that the maximum 
permissible current will not flow through the armature 
when the rheostat arm touches the first stud, but only 
when contact is made with the second or third stud. 

99. Speed Regulation. — A shunt-wound motor under 
constant impressed terminal voltage will have an approxi- 



MOTORS. 233 

mately constant speed. It will decrease somewhat as the 
load on the machine increases. The principal cause of this 
speed variation under varying load is the change of the 
armature resistance drop, and it is therefore desirable that 
the resistance of the armature be small. The change of 
speed, with a fixed setting of the field rheostat, from full 
load to no load, expressed in terms of the speed at full load, 
is called the speed regulation of the motor. For example, 
the speeds of a shunt motor at no load and at full load are 
S6d and 825 rev. per min. respectively. Consequently its 
speed regulation is 

860-825 

= .0425, or 4 J per cent. 

025 

The maintenance of a strictly constant speed necessitates 
the manipulation of a field rheostat, that is, the adjustment 
of a device external to the motor itself. Speed regulation 
is to be distinguished from speed control. The former in- 
dicates the speed changes inherent in the machine, whereas 
the latter means adjustment for various desired speeds. 

100. Characteristic Curves of Shunt Motors. — The char- 
acteristic curves of a motor include curves of speed, effi- 
ciency, current input, and torque, in terms of the H.P. 
output of the machine. Such curves for a 7.5-H.P., 230- 
volt General Electric Company Type CQ motor are shown 
in Fig. 146. 

A shunt motor when started cold on no load quickly 
arrives at a speed which {hen. gradually rises to a maximum. 
The gradual heating of the field coils increases their resist- 
ance. This allows less current to flow in them, and the 
resulting magnetic flux is less. Therefore the armature 
must rotate faster to generate the same counter E.M.F. 



234 



DYNAMO ELECTRIC MACHINERY. 



The efficiency of a motor is the ratio of the mechanical 
output to the electrical input. The determination of the 
output may be made directly by experiment, or it may be 



120 



£ 40 







































































EFFICI 


;ncy 


















SPEED 




















f 


/4 




















<°' 













































































































H. P. OUTPUT 

Fig. 146. 



50 1000 



30^ COO 



found from the measurement of the losses. The latter 
method is to be preferred because of its greater accuracy. 
The various losses may be obtained as in Chap. VI. Thus, 
the efficiency is 

746 H.P. 



£ = 



746 H.P. +(P h +P.+Pa+P p + Pf + P M +PJ' 



(§69) 



The efficiency at any load should be determined at the ulti- 
mate temperature assumed under continuous operation at 



MOTORS. 



235 



that load, referred to the standard engine-room temperature 
of 25 C. 

In the direct determination of motor output, any con- 
venient load may be placed on the machine. For the 
smaller motors a Prony brake may be used, the strap-brake 
being a convenient form, Fig. 147. The power absorbed 
in watts is expressed as 



Output 



2xrV(P-P f )746 
33000 



where r is the radius of the pulley in feet, V is the speed 
in rev. per min., and (P —P') is the difference of the two 
scale readings in pounds. 

For large motors the load usually consists 
of generators, the output of which may be 
absorbed by suitable resistances. If the 
generator be of proper voltage the current 
therefrom may be returned to the sup- 
ply circuit. This method of loading a 
motor, called the loading -back method, 
results in a considerable saving of power, 
since the net power taken from the mains 
is only that required to supply the losses 
of the two machines. The amount of 



Fig. 147. 

load is regulated by changing the field current of the 
generator. 

The efficiency of a shunt motor at full load can be esti- 
mated from the data stamped on the name plate of the 
machine. Thus, for the following data : 




H.P. 

Volts 



20 
120 



Amp. 
R.P.M. 



150 
925 



236 DYNAMO ELECTRIC MACHINERY, 

the efficiency at full load is 
746 X 20 



120 X 150 



.83, or 83 per cent. 



101. Industrial Applications of Shunt Motors. — The 

design of motors differs frequently in many details from 
that of generators, especially if the motors are to be di- 
rectly coupled to the machines they drive. These points 
of difference are principally in the construction of the 

frame, bearing supports 
and shafts. Often motors 
must be placed out of 
doors or where they are 
exposed to dust, chips, 
etc.; in such cases they 
should be of the enclosed 
type. 

Fig. 148 depicts a back- 
geared motor driving a 
Hamilton drill press. This 
type of motor is desirable 
for slowly moving ma- 
chines, since it permits of 
the usual high armature 
speeds. A protecting 
guard surrounding the 
gear and pinion is usu- 
ally furnished to prevent 

Fig. 148. . , 

accidents. 

A Crocker -Wheeler adjustable-speed motor directly 

geared to a 36-inch lathe is shown in Fig. 149. Motors 

so situated are usually of the open type, but they are 




MOTORS. 



237 




Fig. 149. 




Fig. 150. 



2 3 8 



DYNAMO ELECTRIC MACHINERY. 



sometimes provided with gridiron covers and gauze to give 
better protection against dirt. 

The costs per hour of operating machine tools driven by 
individual motors are given in the following table, the data 
representing conditions such as obtain in large machine 
shops. Fixed charges include interest and insurance on 
investment in buildings and equipment, variable charges 
include repairs and renewals, and salaries include cost of 
management, engineering, labor, etc.; these charges are 
apportioned among the various machines. 







Hourly 


Operating Expense i> 


• Dollars. 




t/i 






H 
in 

O 


z 


X 




Type of 
Machine. 


SIZE. 


w 


X 

< 


2 ° 


in 
W 


u 

°8 




H J 

28 











u 

Q 
a 

X 

E 


< x 


< 
►J 
< 

• T 5 


W 
H 
Z 


u 

X ft. 

- 
w 

Q 




H 

o> 

O 
U 


TOTAL. 


Vertical 


4o"-6o" 


02 


• 25 


•05 


•05 


.Ol 


0-53 


Boring 


72"-IOO" 


04 


•45 


•25 


.08 


.08 


.OI 


O.9I 


Mills 


io'-i4' 


05 


.80 


.40 


•15 


• 15 


.02 


i-57 




i6' -24' 


08 


2 .00 


1 .00 


.30 


•3° 


•°3 


3-7i 


Radial 


5' 


02 


•30 


.20 


■°3 


•03 


.OI 


0-59 


Drills 


io' 


04 


.60 


•35 


.09 


.09 


.OI 


1. 18 


Engine 


3o"-4o" 


02 


•25 


. 12 


.04 


.04 


.01 


0.48 


Lathes 


40"-6o" 


03 


5o 


• 2 5 


. 10 


.10 


.01 


0.99 




3 6"- 5 6" 


04 


•55 


•3° 


•05 


•05 


.01 


1 .00 


Planers 


7' -10' 


06 


1 .10 


.60 


•15 


•!5 


.02 


2.08 




12' -14' 


J 5 


2.60 


1 .40 


• 2 5 


•25 


•03 


4.68 



Power for machine tool operation may be furnished 
either by individual motors or from a line shaft. The ini- 
tial investment for line-shaft drive is usually less than for 
individual motor drive, but the latter is conducive to in- 
creased production. Heavier cuts are possible and the time 
for a given operation is shorter with individual motors. 

Fig. 1 50 illustrates the operating mechanism of the Otis 



MOTORS. 239 

Traction Elevator, which consists essentially of a slow- 
speed shunt-wound motor, a sheave, and a brake pulley, the 
latter enveloped by a pair of powerful spring-actuated and 
electrically released brake shoes, all compactly grouped 
on a heavy iron bedplate. The armature shaft serves as a 
support for the elevator car and counterweight, and on 
it are mounted the sheave and brake pulley, the drive 
between the armature spider and sheave being effected 
through the engagement of projecting arms on each cush- 
ioned by rubber buffers. A controller is used for accel- 
erating and retarding the car. The control equipment is 
so designed that the cars are automatically and gradually 
retarded and brought to rest at the upper and lower termi- 
nals of travel, an operation which is entirely independent 
of the position of the car controller. Apparatus of this 
kind is installed in the Singer and Metropolitan towers in 
New York City, and enables one to reach the fortieth 
floors of these buildings from the street level in less than 
one minute. 

Series Motors. 

102. Series Motors. — As the current traversing the field 
windings of a series-wound motor is the same as that which 
flows through its armature, the field strength of such a 
machine will vary with the load placed upon it. Torque, 
being proportional to the product of the magnetic flux and 
the armature current, § 92, will vary approximately as the 
square of the current taken by the motor. This is true for 
a series motor with an unsaturated magnetic circuit, but in 
practice the magnetic circuit is designed to approach satu- 
ration near the rated output, and consequently the torque 
exerted varies to a smaller extent than the square of the 
current. 



240 DYNAMO ELECTRIC MACHINERY. 

The speed of a series motor in revolutions per minute is 
„ (E-IR)6o 8 /e . 

V ~ 2 fi*S IO? (§93) 

where E is the impressed E.M.F., R is the combined re- 
sistance of armature and field windings, p is the number of 
pairs of poles, <£ is the total flux per pole, and 5 is the 
number of armature conductors in series between brushes. 
An inspection of this expression shows that with increased 
load the numerator will be but slightly altered because R 
is small, and that the denominator will be considerably 
increased since <J> varies with the current. Consequently 
the speed of the motor decreases as the load increases. 

The speed of the armature of a series motor will be such 
that the counter E.M.F. generated at that speed will re- 
duce the current to a proper value, so that the total power 
consumed will be equal to the sum of the motor output 
and the losses. In a shunt-wound motor, a very small 
variation of speed is sufficient to compensate for a wide 
variation of load. With decrease of load both shunt and 
series motors speed up and generate a higher counter 
electromotive force. The resulting decrease of current 
causes, in the series machine, a weakening of the magnetic 
field, and as a consequence additional speed is required to 
maintain this E.M.F. Thus a small variation in load on a 
series machine results in a wide change of speed. 

The exertion of a large torque at low speeds and a small 
torque at high speeds results in a rather uniform energy 
consumption, for power output equals the product of torque 
and angular velocity. For this reason the series motor is 
particularly suitable for traction and for the operation of 
cranes and of rolling mills. A series-wound machine can 



MOTORS. 241 

be used on either a constant-current circuit or on a con- 
stant-potential circuit ; but a series motor is seldom run on 
a constant-potential circuit unless it is directly or very 
solidly coupled with its load. If connected by means of a 
belt, and if the belt should break or slip off, the motor 
would speed up indefinitely and cause the armature to fly 
to pieces. The series motor, therefore, cannot be run at 
no load and rated voltage. This difficulty does not pre- 
sent itself when series motors are operated on constant- 
current circuits, a practice no longer in vogue. 

103. Characteristic Curves of Series Motors. — The char- 
acteristic curves of a 5-H.P., 220-volt, back-geared, series- 
wound motor are shown in Fig. 151, and include curves of 
current input, torque, speed and efficiency, plotted in terms 
of the horse-power output. The high speeds attained when 
the motor is under light loads are clearly indicated by 
the speed-output curve. Frequently curves of torque in 
terms of speed are used, especially in the selection of motor 
capacity for electric cars or locomotives. Fig. 152 depicts 
such a curve for the 5-H.P. motor mentioned above. 

If a series motor be at rest and the circuit be closed, an 
enormous rush of current will occur, giving a tremendous 
torque. Destructive heating and sparking would probably 
result. To prevent damage it is therefore necessary, in 
the operation of these motors, to insert at the start a series 
resistance which may be cut out after the speed has risen 
enough to give a sufficient counter E.M.F. In practice 
controllers are used for this purpose. 

104. Railway Motors. — Series motors operating on con- 
stant-potential circuits of from 500 to 600 volts furnish a 
very satisfactory motive power for the propulsion of trolley 
street cars and electric railway motor cars. This type of 



242 DYNAMO ELECTRIC MACHINERY. 



tr z 























































EFFIC 


iNCY 










































w 




/? 




















n? 




































X^ 






















































1 










i 







H. P. OUTPUT 

Fig. 151. 



50 















































K 






















ffi 
■ii 

3 
O 
IE 

o 

K 

10 










































































































1200 1600 

REV. PER MIN. 



Fig. 152. 



MOTORS. 243 

motor has been developed to a high degree of perfection 
during recent years, and is reasonably well fitted to meet 
the many and severe requirements of railway service. Re- 
cent improvements are directed to reliability rather than to 
increased efficiency. It is not unusual for modern railway 
motors to be in service for a year or more or to have trav- 
eled 60,000 miles without overhauling. 

A railway motor must be mechanically strong to with- 
stand the continual strains to which it will be subjected 
when in service. Poor roadbed, defective switches, snow- 
covered tracks, etc., are conditions met with in railway ser- 
vice. Railway motors are also subject to abuse at the 
hands of the motormen. The series resistance is often cut 
out too rapidly, before the car has an opportunity to accel- 
erate. As a result there is an enormous current flow and 
a large torque exerted with little speed. This severely 
strains the motor and is particularly liable to disturb the 
armature windings. Railway motors are of weatherproof 
construction, being totally enclosed to guard against the 
intrusion of water, slush and mud. 

Fig. 153 illustrates the box-type frame of a No. 134 
Westinghouse railway motor for the heavier class of inter- 
urban service. There are four poles built up of soft steel 
punchings assembled and riveted together between wrought - 
iron end plates and bolted to the motor frame. The field 
coils are straight and are formed of copper strap wound on 
edge, the individual turns being insulated from each other 
by asbestos. The coils are insulated by several tapings 
and impregnated with an insulating compound to render 
them impervious to moisture. They are held in place 
independently of the poles by brass hangers which are 
bolted to the motor frame. 



244 



DYNAMO ELECTRIC MACHINERY. 



The armature bearings of this motor are carried in hous- 
ings which are firmly clamped into bored seats in the frame. 
The bearing at the commutator end is 3f inches in diame- 
ter and 10 inches long. One end of the motor frame con- 
tains bearings which run on the wheel axle and keep the 




Fig. 153. 



pitch circle of the armature shaft pinion always tangent to 
the pitch circle of the gear which is mounted on the axle. 
These bearings are 1 1^ inches long and are furnished for a 
maximum axle diameter of 6 inches. The bearings, both 
for armature shaft and for axle, consist of solid bronze shells 
lined with babbitt metal soldered to the bronze, and are 



MOTORS. 245 

arranged for oil-saturated waste as lubricant. Large pock- 
ets are provided for the waste which is in contact with the 
shaft, and the oil is led up to the waste from below. The 
openings in the bearing shells are usually 60 % of the total 
length of the shell and 80 degrees wide. 

The armature is built up of thin soft-steel laminations 
mounted on a spider together with the commutator. Open- 
ings in the laminations and spaces between groups of them 
provide for thorough ventilation by means of the air drawn 
in at the ends and passing out against the field windings. 
The winding consists of formed single-turn coils made in 
two parts, the lower and upper halves being connected at 
the rear of the armature by soldered copper clips. The 
winding is insulated with mica and sealed by linen tape 
followed by dipping in varnish to render it oil-proof. The 
winding is firmly secured in place by steel wire wound around 




Fig. 154. 

the core and over the end connections. Fig. 154 shows the 
armature and commutator mounted on the shaft. The 
diameter of the armature is 17J inches and that of the 
commutator is 14! inches. Brush holders, Fig. 155, for 
this motor are supported by two steel studs which are 



246 DYNAMO ELECTRIC MACHINERY. 

secured to the motor frame by means of clamps, as 
shown. 

The railway motor described in the foregoing has a 
nominal rating of 160 H.P. based on a one-hour run with 
a temperature rise not exceeding 75 C, as thermometri- 
cally measured, in any part of the winding above the sur- 
rounding air taken at 25 C. An equipment comprising 




Fig. 155. 

two such motors would propel a car weighing, without pas- 
sengers and electrical equipment, 25 tons, over a level 
track, and maintain a schedule speed of 30 miles per hour 
with stops two miles apart. These figures are based upon 
a gear ratio of 24-53 an d 33-inch car wheels. 

The performance curves of this railway motor at 500 
volts are given in Fig. 156. The usual torque and rev.- 
per-min. curves of motors are replaced in railway work by 
curves of tractive effort (i.e., force exerted at the base of 
the car wheels) and speed in miles per hour. Knowing 
the gear ratio, gear efficiency, /?, and car-wheel diameter, 



MOTORS. 



247 



D inches, this conversion can be effected by means of the 
following expressions : 

Lbs. Tractive Effort = 



no. gear teeth 24 8 _ . ., x . 

• -~ . J^- x Torque in lbs.-ft. 

no. pinion teeth D 



Miles per Hour 



2 7z 60 Rev. per min. X Torque in lbs. ft. 
5280 X Lbs. Tractive Effort 



The continuous capacity of this motor is given as 120 
amperes at 300 volts and no amperes at 400 volts. 















G 


EAR RA 
33VV 


no 24 ■ 

HEELS 


53 


































































EFFIC 
WITH 


ENCY 
SEARS 




















^ 


















4y 




















7 
























_gPEED 














* 








































































20C0 100 



1000 60 



Fig. 156. 



A motor for railway service, very similar in design to 
the one just described, is the GE-216 made by the Gen- 



248 DYNAMO ELECTRIC MACHINERY. 




MOTORS. 



249 



eral Electric Company, and shown in Fig. 157. It is a 
4-pole motor provided with an equal number of commutat- 
ing poles, the latter being conducive to better commutation 
during the acceleration period. The one-hour rating of 
this motor is 50 H.P. at 600 volts. 

The gear case rides with the motor and is fastened to the 
magnet frame at three points in order to eliminate vibra- 
tion. The case is made of malleable iron and constructed 
with strengthening ribs to prevent cracking. 

Some operating characteristics and constructive data of 
550-volt railway motors are embodied in the curves of Fig. 
158. Curve A represents the efficiency of the various sizes 



5000 80 10000 



10 



100 






































(i 






z 

tso 

Ul 

z 








B^ 


























A 








111 
0. 














E 






















iT" 































40 80 120 

H. P. OUTPUT 



Fig. 158. 



of motors at normal load, curve B shows the radial length 
of the air-gap between armature and field poles, curve C 
gives the peripheral speed of the armature in feet per 
minute, curve D indicates the weight of the motor per 
horse-power, and curve E shows the number of ampere- 
turns per field coil at normal load current. 



250 DYNAMO ELECTRIC MACHINERY. 




MOTORS. 251 

The manner of suspending the motors from the trucks 
is a matter of considerable importance. One end of the 
motor frame contains bearings which run on the wheel 
axle, and the other end or the sides are provided with lugs 
for attachment to a heavy bar which is supported by springs 
on the truck frame. Figs. 159 and 160 show two methods 
of motor suspension. 

At present a few interurban railways are in operation 
in this country upon which 1200- and 1400-volt series 
motors are used. The design of these motors is not mate- 
rially different from that of the 600-volt type, but particu- 
lar attention is directed to the avoidance of commutation 
difficulties. 

105. Railway Motor Control. — The function of a rail- 
way controller is to allow the motors to start from rest and 
to accelerate to full speed, this operation being performed 
with moderate uniformity, due consideration being given to 
the durability of the apparatus and to the comfort of pas- 
sengers. Two general methods for accomplishing this 
result are in use : the rheostatic, and the series-parallel 
method. 

-O 1 0-L0 2 cJ-o 3 o-i-0 4 (J J_ 

GROUND 

Fig. 161. 

In the rheostatic method for use with one or more motors, 
resistance is placed in the motor circuits, which can be ad- 
justed to regulate the impressed electromotive force. A 
scheme of connection for a rheostatic railway controller is 
given in Fig. 161. The change of the resistance in the 
motor circuit is accomplished by short-circuiting successive 



252 DYNAMO ELECTRIC MACHINERY. 

portions of it by closing switches i, 2, 3 and 4 in the order 
named. This method is infrequently employed although 
simple, because the loss in the regulating resistance does 
not permit of economical operation. 

The series-parallel method of motor control is exten- 
sively used for equipments with two (or any multiple of 
two) motors. The speed of the car is regulated by first 
placing the two motors and a resistance in series, and then 
cutting out the resistance step by step until the motors 
are operating in series on full voltage. Since, with all the 
resistance cut out, there is no unnecessary I 2 R loss, this is 
called a running connection, and the controlling mechanism 
is said to be upon a running point. To further increase 
the speed, the motors are placed in parallel with a resist- 
ance in series with both. This resistance is then cut out 
step by step until the motors are each operating on full 
voltage. This, again, constitutes a running connection. 

The connections of a series-parallel controller are more 
complex than those of the rheostatic type, since the mat- 
ter of transition from the series to the parallel positions 
demands attention. During this period one motor may 
be shunted or short-circuited, the motor circuit may be 
opened, or the full current may be maintained through all 
motors. A scheme of connections illustrating the latter 
type of series-parallel control is shown in Fig. 162. The 
controller performs the following operations : switches 
A and B are closed, thus placing both motors and all the 
resistance in series ; switches 1 to 7 are closed consecu- 
tively and then switch C is closed; followed by the opening 
of switches 2 to 7 and B ; switches a and b are closed ; thus 
two currents will flow through switch C in opposite direc- 
tions, one from the trolley through the motors to ground 



MOTORS. 



253 



and the other through the resistance to ground. If the 
resistance be properly proportioned no current will pass 
through switch C, and this may be opened, thus placing 
both motors with resistance directly across the line. 
Switches 2 to 7 are then closed progressively as before, 
after which the motors are operating in parallel without 
resistance. This scheme is therefore desirable in that 
no motor is subjected to sudden increased voltage nor is 
the circuit opened at any time. 



0-L0 ,-Ay 
a a 




B b 



1: 




5 O-i-O 6 



Fig. 162. 



When four motors are installed on a car, they may 
first be connected in series, then each pair in parallel 
with the two groups in series, and finally all connected in 
parallel ; this is known as the series, series-parallel, parallel 
method. 

The manipulation of the various switches may be accom- 
plished directly by hand or through the intervention of an 
auxiliary control. In the former system the connections 
are made by a motorman who moves a handle at the top of 
the controller on the car platform. This causes the rota- 
tion of a vertical cylinder and permits of the successive 
connection of various contact studs thereon with stationary 
fingers. Such a controller, made by the Westinghouse 
Electric and Manufacturing Company, is shown, with the 



254 



DYNAMO ELECTRIC MACHINERY. 



cover removed, in Fig. 163. In this controller for series- 
parallel operation there are seven controlling points in the 
series position and six in the parallel position ; during the 
transition from one position to the other one of the motors is 
short-circuited. The wires from the trolley, from the 




Fig. 163. 

motors, and from the different terminals of the resistance 
grids are brought up under the car to the proper fingers. 
A smaller cylinder, moved by a reversing lever, is situated 
to the right of the main cylinder. This has contact 
pieces which are arranged so as to enable the motorman 
to reverse the direction of rotation of both motors or 
to cut them out entirely. Interlocking devices are sup- 



MOTORS. 255 

plied so that the reversing handle cannot be moved unless 
the controlling handle is in such a position that connec- 
tion with the trolley is broken. The controlling handle also 
cannot be moved if the reversing handle is not properly 
set either to go forward or to go backward. The reversing 
handle cannot be removed from the controller, save when 
the smaller cylinder is in the position that cuts out both 
motors. As serious arcs are liable to develop upon break- 
ing a circuit of 500 volts, the contact pieces and fingers are 
separated from adjacent ones by strips of insulating mate- 
rial which are fastened to the inside of a separate cover. 
Such arcs are effectively disrupted by the field of an electro- 
magnet, which is an essential part of controllers for motors 
of large size. 

In operating an electric car, the power should never be 
turned off by a slow reverse movement of the controller 
handle, as destructive arcs are liable to occur upon a slow 
break. To lower the speed of a car, the power should be 
completely and suddenly shut off. Before the car has 
slackened its speed too much the controller handle can be 
brought up to the proper point. 

The system of motor car control in which the various 
switches are operated by an auxiliary circuit is called the 
multiple-unit control, since it is designed primarily for 
the operation of several cars coupled together in a train 
from any controller on it. The control apparatus for each 
motor car consists essentially of a series-parallel motor con- 
troller and two master controllers. The motor controller 
is composed of a number of electrically operated switches 
or contactors which close and open the various motor and 
resistance circuits, and a separate electrically operated re- 
versing switch which governs the direction of movement 



256 DYNAMO ELECTRIC MACHINERY. 

of the car. This apparatus is usually placed underneath 
the car. Both the contactors and the reverser are operated 
by solenoids, the current to which is varied by the master 
controller. The latter is considerably smaller than the 
ordinary street-car controller, but is similar in appearance 
and method of operation. A cable which connects each 
master controller with the motor controllers runs the en- 
tire length of the train, the connections between cars being 
made by suitable couplers. Current for the master con- 
trol is taken from the line through whichever controller 
the motorman operates, the amount being about 2\ am- 
peres for each equipment of 400 H.P. As the motor 
controller is connected to the train auxiliary circuit, any 
master controller on the train will simultaneously operate 
corresponding contactors on all the motor cars and estab- 
lish similar motor connections on them. The connections 
of the Sprague-General Electric multiple-unit control sys- 
tem are shown in Fig. 164. 

If the current supply be momentarily interrupted, the 
motor control switches automatically return to the "off" 
position, and upon the restoration of the power supply 
the same connections are again progressively made that 
existed immediately preceding the interruption. To 
avoid accidents which may occur through the physical 
disability of a motorman, master controllers are sometimes 
arranged with a button on the handle which must be 
kept down in order to keep the auxiliary control circuit 
closed. 

The multiple-unit control system of the Westinghouse 
Electric Company differs from the preceding in the method 
of actuating the contactors and reversers. Compressed 
air is used for this purpose, the necessary valves being 



MOTORS. 



257 




258 DYNAMO ELECTRIC MACHINERY. 

operated electrically by a master controller from a storage 
battery. 

106. Motors for Automobiles. — For electric automo- 
biles the series-wound motor is invariably employed. A 
storage battery of 40 or 44 cells is the customary source 
of power for these motors. The use of these cells affords 
a convenient and economical means of speed control. In 
the case of a single motor, for the first controller notch, 
the cells may be connected in four-series groups of 10 or 
1 1 each, giving about 22 volts, the four groups being con- 
nected in parallel. Other notches would correspond to 
other series-parallel combinations, and finally the last and 
highest speed notch would correspond to a connection of 
all the cells in series. By this arrangement one cell is 
used just as much as any other, and they are discharged 
at equal rates. As the voltage supplied to the motor is 
varied without recourse to a series regulating resistance, 
there is no resistance loss in starting or running at less 
than full speed. 

Frequently two motors are used, one on each of the 
two driving wheels ; this arrangement allows independent 
rotation of the wheels on turning curves, while if only one 
motor be used some form of differential gear must be em- 
ployed to allow the vehicle to make sharp turns. But the 
efficiency of one motor is in general greater than the effi- 
ciency of two motors of half the rating, and the gain in 
efficiency by using one motor more than balances the cost 
and complication of a differential gear in the case of light 
vehicles. 

It is general practice to rate automobile motors at 75 
volts, or at 37J volts if two are used in series and con- 
trolled as a single motor. Since the voltage of 40 or 44 



MOTORS. 259 

cells of battery in series can fall to 75 volts without injury, 
this is the lowest pressure on which the motors will be 
expected to run for any length of time at full speed. 
Hence this voltage is used as the basis for rating. For 
the best motors the rating is such that a temperature rise 
of 50 C. will not be exceeded on an indefinite run. A 
motor so rated will carry 100 per cent overload for a half 
hour without overheating or damaging the insulation. 

Although the voltage of these motors is somewhat low 
for the use of carbon brushes, the necessity of reversal of 
direction and the liability of sparking on overload make 
their use desirable. Soft carbon brushes of low resistance 
can, however, be obtained, and they are to be recom- 
mended. 




Fig. 165. 

Fig. 165 illustrates a motor which is used on automo- 
biles and manufactured by the Eddy Electric Manufactur- 
ing Company. The armature winding consists of formed 
coils which are cross-connected, and therefore only two 
brushes are required. These brushes are made accessible 
by providing a window in the end plate. A pinion which 



260 DYNAMO ELECTRIC MACHINERY. 

is mounted upon the armature shaft meshes with an inter- 
nal gear on the wheel of the vehicle. 

107. Motors for Rolling Mills. — For many kinds of 
mill work requiring great torque, reversibility, and wide 
variation of speed, the series motor is well adapted. The 
shocks and jars which such motors must withstand are 
very severe because the load conditions are heavy and 
intermittent, and therefore they must be of particularly 
strong construction. Mill motors must be totally enclosed 
to guard against dust and small particles of metal, and con- 
sequently must be amply designed so that their tempera- 
ture elevation will not be excessive. These motors usually 
operate in both directions and therefore the brushes can 
have no lead. Sparkless running is insured by employing 
large air-gaps. 

The Crocker- Wheeler Company manufacture form W 
motors for rolling mills in sizes ranging from *]\ to 200 
H.P. for 220 volts, one of which is shown in Fig. 166. 
They are four-pole machines, and the frames are divided 
horizontally, the upper half being provided with two hand 
holes for access to the commutator and brushes. 




Q 



TWO HIGH 
MILL 

THREE HIGH 
MILL 



Fig. 166. Fig. 167. 

A rolling mill may be two high or three high, as indi- 
cated in Fig. 167. In the latter the center roll rotates 



MOTORS 261 

constantly in one direction, while the other two rolls revolve 
constantly in the opposite direction. Thus a piece of steel 
can pass through the lower set to the right, then be raised 
on a table and pass through the upper set of rolls to the 
left, and continuing in this way. In the two-high mill the 
steel must pass through the rolls in both directions and 
consequently the motor driving such a mill must be capa- 
ble of reversal. Difficulty is encountered in designing 
motors which are to reverse their direction quickly because 
of the large moments of inertia of the armatures and 
rolls. Sometimes two or three armatures of smaller 
diameter than an equivalent single armature are placed 
upon one shaft, thus obtaining a smaller radius of gyra- 
tion. 

A mill motor has been built by Messrs. Siemens weigh- 
ing 235 tons, the armature weighing 74 tons, and it is 
capable of exerting a torque of 650,000 lbs.-ft. up to a 
speed of 60 rev. per min., thus corresponding to over 7000 
H.P. output. This mill has been in operation for some 
time, and it is found possible to reverse its direction 28 
times per minute from a speed of 60 rev. per min. in one 
direction to an equal speed in the other direction. 

The coupling between a motor and a rolling mill should 
be such that if the roll breaks obliquely the resulting end 
thrust will not damage the motor. Some form of shell 
coupling should be used between the spindle and the motor 
shaft. 

108. Crane Motors. — Series motors for operating cranes 
or hoists are generally equipped with a brake attachment 
so that the load may be held after raising it. Brakes are 
of two types, friction brakes and dynamic brakes. Fric- 
tion brakes are made in a number of ways, one type of 



262 



DYNAMO ELECTRIC MACHINERY. 



which is depicted in Fig. 168. This shows a spring-actu- 
ated shoe brake which is kept normally in engagement but 
is released when current is supplied to the solenoid. An- 
other form is the band brake, but this is mostly used with 
non-reversing motors, although some forms are applicable 
to reversing motors. 




Fig. 168. 



Dynamic braking is accomplished by connecting the 
motor to operate as a generator which will deliver energy 
to some local circuit or return it to the supply circuit. 
Such brakes are generally supplemented by friction brakes 
which become operative when the motor comes to rest and 
the dynamic braking ceases. The controller for dynamic 
braking is arranged to connect the armature in a closed 
circuit containing an adjustable resistance, or to the line. 
It is desirable first to connect the series field with resist- 
ance across the line wires to insure the motor building up 
as a series generator. Then the motor is disconnected 



MOTORS. 



263 



from the line, leaving the motor armature and field in cir- 
cuit with the resistance. In Fig. 169 these connections 
are shown respectively at a and b. 

FIELD FIELD 

p^W" 1 1 p/TflTOffJ^ 1 

=— L< j—www-^ 1-4 Vaaa/v 

\_y RESISTANCE \ S RESISTANCE 

a b 

Fig. 169. 

A controller for crane motors made by the Cutler- 
Hammer Manufacturing Com- 
pany, intended for either hoist 
or travel duty, is shown in Fig. 
170. The resistances are of 
the cast-iron grid type and are 
supported by iron rods over 
which mica tubes are previously 
placed. Controllers for hoist 
duty are provided with higher 
resistance than those for travel 
duty so as to insure good 
speed control under light 
loads. 

109. Compound -wound Mo- 
tors. — In a compound-wound 
motor the magnetomotive force 
of the shunt winding may as- 
sist or oppose the magneto- 
motive force of the series 
winding, depending upon the 
manner of connection. In the 
first case the machine is called m s- 170 

a compound motor and in the latter a differential motor. 




264 DYNAMO ELECTRIC MACHINERY. 

The magnetic field of a compound motor becomes more 
intense with increasing load, and consequently the speed 
decreases ; the amount of speed decrease will depend upon 
the relative magnetomotive forces of the two windings. 
At light load there is always a definite field strength, due 
to the shunt winding, and therefore the speed of the motor 
cannot exceed a predetermined value. For heavy inter- 
mittent loads, such as in operating rolling mills, hoists, ele- 
vators, etc., compound motors are much used, because they 
can exert a powerful starting torque and yet the speed will 
not be excessively variable under changes of load. Such 
motors may be safely belted to machine tools in operating 
them. 

Differential motors may be designed to run at almost con- 
stant speed by properly proportioning the series and shunt 
windings so that the magnetic field becomes weaker as 
the load increases. A powerful starting torque cannot be 
obtained from this motor inasmuch as the large starting 
current in the series winding greatly decreases the field 
strength. If such motors be suddenly started under load 
their direction of rotation may reverse because the series 
winding has a lower inductance than the shunt winding ; 
hence in starting differential motors the series winding at 
first should be automatically short-circuited. These motors 
are rarely used in practice, as improvements in the design 
of shunt motors have given the latter good speed regula- 
tion, so there is no need of resorting to differential motors 
with their objectionable features. 



PROBLEMS. 265 

PROBLEMS. 

1. The armature core of a 4-pole motor has 41 slots, each 
containing 24 conductors. At full load the wave-wound arma- 
ture takes 20 amperes at 500 volts, and a flux of 2,100,000 
maxwells enters the armature core per pole. What torque is 
developed at full load ? 

2. If the armature mentioned in the preceding problem 
revolve at 640 rev. per min , determine the horse-power devel- 
oped at full load. 

3. The armature resistance, including that of brushes and 
brush contacts, of a 250-volt, 6-pole motor is .0079 ohm. The 
armature is lap-wound and has 570 conductors. What is the 
flux per pole entering the armature when the latter runs at 400 
rev. per min. and takes 660 amperes at full load ? 

4. A 25-H.P., 220-volt shunt-wound motor takes a current 
of 98 amperes at full load. The armature resistance is 0.090 
ohm. With a maximum allowable current intake of 1 40 amperes, 
calculate the number of steps required in a starting rheostat for 
this motor. 

5. What are the resistance values of the various steps of the 
starting box mentioned in the previous problem ? 

6. From the following name-plate data of a shunt motor 
determine its regulation and full-load efficiency : 

H.P. = 50, Volts = 250, Amperes = 170, 

R.P.M. = 420 at no load, R.P.M. = 400 at full load. 

7. What is the average pull in pounds on each of the 372 
armature conductors of the motor cited in the foregoing problem 
at full load if the conductors are situated 9 inches from the axis ? 

8. The motor of an electric car having 33-inch wheels, when 
traveling at 25 miles per hour, exerts a torque of 550 pounds (at 
one foot radius from the center of the armature shaft). If the gear 
ratio is 26 to 60, and the efficiency of the gears is 97%, determine 
the tractive effort at the base of the car wheels, the horse-power, 
and the number of revolutions of the motor per minute. 



266 DYNAMO ELECTRIC MACHINERY. 



CHAPTER VIII. 

DYNAMOTORS, MOTOR-GENERATORS, BOOSTERS, AND 
STORAGE BATTERIES. 

no. Dynamotors. — Dynamotors are transforming de- 
vices combining both motor and generator action in one 
magnetic field, with two armatures or with an armature 
having two separate windings. They are generally supplied 
with commutators, one at each end, which are connected to 
the two windings respectively. Either winding of the ar- 
mature may be used as a motor winding, and the other as 
the generator winding. These machines occupy the same 
position as regards direct-current practice as is occupied 
by transformers in alternating-current practice. That is, 
they enable one to take electrical energy from a system of 
supply at one voltage, and deliver it at another voltage to 
a circuit where it is to be utilized. They cannot, however, 
be constructed so as to operate with the same high 
efficiency as a transformer does. As the currents in the 
two armatures flow in opposite directions, and the machines 
are so designed as to have practically the same number of 
armature ampere-turns when in operation, there is practi- 
cally no armature reaction. The field, therefore, is not 
distorted so as to require a shifting of the brushes upon a 
change of load. These machines are more efficient than 
motor-generators, which will be described later, as they 
have but a single field magnet. They cannot be com- 
pounded so as to yield a constant E.M.F. at the dynamo 
end, for, while a cumulative series coil would tend to raise 



DYNAMOTORS. 267 

the E.M.F. at the generator end, there would be a 
corresponding decrease of the speed of the armature, 
due to the increase of magnetic flux. To generate the 
same counter E.M.F. in the motor winding as previously 
existed without the series coil, requires a lower armature 
speed. 

Dynamotors are used in the so-called Teaser system of 
variable speed control of motors which are intended to re- 
ceive energy from supply circuits, that are at the same time 
giving energy to lighting and similar load circuits. For 
instance, they are used in connection with large printing 
presses whose parts have large moments of inertia, and which 
demand an unusually large starting torque to be exerted by 
the driving motor. Sometimes this is as much as six times 
the torque which the motor is called upon to exert at nor- 
mal speed and full load. Since the torque which is exerted 
by a motor is dependent upon the current which flows 
through its armature, while the speed at which this torque 
is applied is dependent upon the pressure impressed upon 
it, it is desirable that the large starting current should be 
taken at a low voltage from some transforming device, such 
as a dynamotor, rather than at the higher and constant volt- 
age of the supply mains. The dynamotor for instance 
may be so designed that the counter E.M.F. of its motor 
winding is five times the E.M.F. induced in its generator 
winding. Consider the dynamotor and main motor to be 
connected to the supply mains as indicated in Fig. 171, the 
field winding of the former being excited by current taken 
directly from the mains, and the negative brush of the 
motor end being connected with the positive brush of the 
generator end. The two armature windings are connected 
in series with a regulating resistance to the supply mains. 



268 



DYNAMO ELECTRIC MACHINERY. 



FIELD COIL 



\AAAA/W\AA/\ 



TEAZER 
ARMATURE 




AA/WWW\) 



Vmaim motor\ 
\armature/\ 



At starting, the main motor, which drives the press and 
which is generally a cumulatively compound-wound motor, 
is supplied with current from the generator end of the dyna- 

motor. The voltage with 
which it is supplied is some- 
what less than one-fifth that 
of the main supply, depending 
upon the magnitude of the 
resistance in series with the 
dynamotor. This low voltage 
permits of the application of a 
proper amount of torque at a 
low speed. Furthermore, the 
drain of current from the sup- 
ply mains is about one-fifth 
Flg * ,7X * that which passes through 

the main motor. By manipulating the dynamo regulating 
resistance, the electromotive force supplied to the main 
motor is raised, and with it the speed. The highest 
speed of the main motor which can be attained by this 
arrangement is such that, when attained, the motor's 
connections may be transferred to the supply mains 
through another series regulating resistance without any 
excessive drain of current from those mains. The amount 
of current which is taken by the main motor as compared 
with the amount of current which is drawn from the supply 
mains may be represented as in Fig. 172. Regulation of 
the resistances and changes of connection are accomplished 
through the aid of a controller. The different speeds are 
secured by the manipulation of a single hand-wheel on the 
controller, and thus the pressman has at his command a 
simple means of manipulating the printing press. 



DYNAMOTORS. 



269 



Dynamotors are also used as equalizers, in connection 
with 3-wire constant-potential distribution circuits, to 




Fig. 172. 



equalize the potential differences between the two outer 
and the neutral wire when one side of the system is carry- 
ing a larger load than the other. For instance, with simi- 




Fig. 173. 



larly wound windings upon its armature, the dynamotor 
would be connected as indicated in Fig. 173 to a system 
supplied by a 220-volt generator. When the system was 



270 



DYNAMO ELECTRIC MACHINERY. 



unbalanced the side having the smaller load would have 
the lesser drop and therefore the higher pressure. The 
winding of the dynamotor which is connected with this 
side would act as a motor winding and drive the armature, 
while the other winding would be the seat of generated 
E.M.F.'s which would tend to raise the pressure of the 
more heavily loaded side. 

Consider that each of the equalizer armature windings 
has a resistance of R ohms and generates E e volts, and 
that the power losses due to hysteresis, eddy currents, and 
friction are Pi watts. Then, with E volts between the 
outer wires, and the load on the two sides balanced, that 
is, no current flowing through the neutral wire, 



and 



E = 2 E e + 2 I R, 
Pi = 2 EI* 



(0 

(2) 



where / represents the current in each armature winding. 
If, however, the load be not balanced, and the neutral wire 
carry a current I n , and if the armature windings carry 




c 'M 



Fig. 174. 



currents T 1 and I 2 respectively, their directions being as 
indicated in Fig. 174, then 

E= 2E Z +I X R-I 2 R, (3) 

(§ 5) /• = h + I* (4) 



DYNAMOTORS. 271 

and, as the iron and friction losses remain unaltered upon 
the changed conditions of load, and since the power ex- 
erted by the motor element equals the power output of 
the generator element plus the frictional and iron losses, 

EJ, = E e I 2 + P, (5) 

Dividing by E e and using (2), 

A = ' 2 +^=' 2 + 2 A- (6) 



E 



Substituting (6) in (4) and solving 

h^f-h (7) 

and i t = ^+ /„. (8) 

From (3), (7), and (8) 

E=2E e + 2l l> R, (9) 

which is the same as equation (i). Therefore the E.M.F. 
generated by each armature winding is not altered by the 
application of the load, under the conditions assumed. 
Half of the unbalanced portion of the load I n is supplied 
by one equalizer armature winding, while the other half 
comes from the main generator after passing through the 
other equalizer armature winding, which at the time is 
operating as a motor winding. The E.M.F.'s generated 
in the equalizer windings are not altered by the applica- 
tion of the load, nor is the speed of the armature. The 
capacity of the equalizer armature, if the losses be disre- 
garded, should be equal to the power represented by the 
maximum unbalanced load, for each armature supplies a 
voltage approximately equal to that of either side of the 



272 DYNAMO ELECTRIC MACHINERY. 

distribution circuit, and carries a maximum current of 
approximately half the maximum carried by the neutral 
wire. The employment of an equalizer enables one to 
dispense with the neutral wire between the point where 
it is located and the main generator, and makes it possible 
to use a 3-wire distributing system in connection with a 
single generator instead of a 2-wire system. 

Assuming the normal pressure per side to be half that 
between outer wires, and since with the maximum unbal- 
anced load the side pressures are respectively 

E AB = E e + I t R, 
and E BC = Ee- I 2 R, 

then the pressure regulations, under this condition, are 
+ IJi/E e and —1' 2 R/E e . The pressure on the loaded side 
of the system therefore drops and that on the unloaded side 
rises, as is the case with two main generators. 

Dynamotors are used in telegraph stations, the motor 
windings being designed for connections to lighting cir- 
cuits, while the generator windings yield pressures suitable 
for telegraphic service. This is often different in the case 
of different machines. These machines are designed to 
take the place of batteries of a large number of gravity cells 
such as were used, in large quantities, a few years ago. The 
cost of operation of a dynamotor for this service is about 
one-fifth of what it is in the case of the gravity cells. The 
space which the machine occupies is much less than that by 
the cells. Dynamotors are to be preferred to batteries also 
on the ground of cleanliness. Their reliability, when supplied 
with electric energy from large city service mains, is equal to 
that of the cells, but this cannot be said in the case of 
small towns. The telephone companies sometimes employ 



MOTOR-GENERATORS. 



273 



dynamotors for the purpose of charging storage cells. 
Such a machine is shown in Fig. 175. With some forms, 
the charging of the cells can go on continuously, they 
being at the same time used for telephonic communication. 
Dynamotors also furnish a convenient and satisfactory 




Fig. 175- 



means of heating surgeons' electro-cauteries. Cautery 
knives take from 3 to 8 amperes at 5 volts, while dome 
cauteries take from 15 to 20 amperes at the same 
voltage. 

in. Motor-Generators. — A motor-generator is a trans- 
forming device consisting of a motor mechanically connected 
to one or more generators. 



274 



DYNAMO ELECTRIC MACHINERY. 



A form in which both the motor and the generator are 
direct-current machines is extensively used in connection 
with 3-wire distributing systems, under the name of bal- 
ancer. If the shunt field coils of the two halves of the 
balancer be connected in series with each other and to the 
outer conductors of the system, and if the armature wind- 
ings be alike, the balancer will operate exactly as would a 
dynamotor when used for this purpose and in the manner 
described in the preceding section. The balancer, however, 
as well as the motor-generator in general, is to be preferred 
to the dynamotor from an operating viewpoint, because the 
absolute independence of the two field magnets and of their 
exciting coils makes possible a considerable variation in the 
speed of the motor element as well as in the voltage of the 
generator element. If the field windings of the two ele- 
ments of the balancer and their armatures be reciprocally 
connected with opposite sides of the system, as indicated 
in Fig. 176, the regulation of the voltage on the two sides 

of the system will be 
improved. The E.M.F.'s 
produced in the two ar- 
matures when one side 
of the system is carry- 
ing a greater load than 
Fi s- x 7 6 - the other will be unlike, 

the motor flux will be reduced and the generator flux will be 
increased. The motor will therefore increase its speed 
and the generator will produce a greater E.M.F. not only 
because of the increased speed of the armature but also 
because of the increased flux from its field magnets. 
With such reciprocal field excitation perfect voltage regu- 
lation cannot be obtained, for the method postulates a 




MOTOR-GENERATORS. 



275 



voltage difference as a basis for its operativeness. Perfect 
regulation may, however, be obtained by the use of com- 
pound-wound elements in 
the balancer. The direc- 
tions of the currents in the 
system and in the exciting 
coils are indicated in Fig. 
177, where the motor and 
generator armatures of the 
balancer are marked re- 
spectively with M and G. The iron of the magnetic cir- 
cuits of the two elements of the compound balancer, under 
normal operation, should not be fluxed to near the point of 
saturation. 




Fig. 177. 




Fig. 178. 



A balancer, for use on 3-wire circuits having a pressure 
of 125 volts per side is shown in Fig. 178. 



276 



DYNAMO ELECTRIC MACHINERY. 



Motor-generators having the armatures of their elements 
wound for unlike voltages are used for maintaining con- 
stancy between the various conductors of multivoltage 
circuits as used in connection with the operation of variable- 
speed shunt motors. Consider such a balancer to be con- 
nected to a circuit and the currents to be as indicated in Fig. 
1 79 and the armature of the lower element normally to gen- 
erate an E.M.F. of E' volts, to have a resistance of R ohms, 




Fig. 179. 

and the armature of the upper element to generate (n —i)E f 
volts and to have a resistance of (n — 1) R ohms resistance. 
Then when the load is so balanced that the middle wire 
carries no current, both elements will act as motors and 
will carry a current I , and the following relations will hold : 

O. (I) 

(2) 



nE f — I nR 



E' = - - LR- 



When, again, the middle wire carries a current I n > as indi- 
cated in the figure, 

E-nE' - /> - i)R + I'R = o, (3) 

and (§5) I t + r = /* (4) 

and, since motor-element output equals the sum of power 
generated and of the frictional losses, 



O - 1) E'I X - E'V + EI 



' 



(5) 



MOTOR-GENERATORS. 277 

but, since nE f is practically equal to E, dividing by £', 
solving for /', and substituting in (4), there results 

(6) 
(7) 



1 n + ° 




Substituting (6) in (4), 




r-U- ')/„ 


-i* 


and therefore, as in (2), 




n 





(8) 

Therefore the main generator supplies approximately 
i/«th the unbalanced load current at full voltage, whereas 
the local mains supply this load current at E/#ths the pres- 
sure between outer conductors. 




Fig. 180. 

In the case of a 4-wire multivoltage distribution circuit, 
using a 3-element motor-generator as a balancer, with cur- 
rents and voltages as marked and directed in Fig. 1 80 the 
preceding equations apply when the symbols are properly 
interpreted. 

Let E t equal the sum of the voltages generated in the 
armature windings of all the elements of the balancer, and 
assume that the different armatures generate AE t , BE t , 
and CE t volts and that their windings have resistances 
of AR, BR, and CR ohms, where A + B+C = i. Then 



278 



DYNAMO ELECTRIC MACHINERY. 



all the armatures carry the no-load current I Qi and each 
armature carries a portion of the unbalanced load-current 
of every other branch than the one to which it is directly 
connected, as well as a portion of the unbalanced load-cur- 
rent of the branch to which it is directly connected. The 
last will flow in a direction opposite to that indicated in the 
figure. According to the foregoing discussion, (6) and (y) y 
the apportioning is as follows : — 



r t -i t + (a 



and 7 2 = 7 + 



/.-.+ 



1)1 A + BI B + 

AIa + (B-i)I b + 

AI A + BI B + (C 



CIc, 
CIc, 
i)I c- 



(9) 



The pressures between successive distribution conductors 
are, therefore, 

E ab = A(E t + I.R), 

E bc = B(E t +I 2 R), 
and E cd = C{E t + I 3 R). (10) 




Fig. 181. 



The equations of (9) and (10) can be extended to embrace 
any number of branches. Balancers having three elements, 
however, satisfactorily meet the requirements of multiple- 



BOOSTERS. 279 

voltage systems for speed control of motors. Common 
branch pressures are 40, 80, and 120 volts respectively. 
By use of such a system a very wide range in speed is 
economically possible. A four-wire balancer for this pur- 
pose is shown in Fig. 181. 

112. Boosters. — A booster is a machine inserted in 
series in a circuit to change its voltage. It may be driven 
by an electric motor, in which case it is termed a motor- 
booster, or otherwise. 

In the distribution of electric energy from a central sta- 
tion, at constant potential, it often happens that excessive 
currents must be supplied at a considerable distance from 
the station or that currents of ordinary magnitude must be 
furnished at very distant points. If the supply potential is 
to be the same at the distant points as at those near by, and 
if the current at all points is to come from the same gen- 
erators, then the cross-section of a feeder to a distant point 
needs to be very great, unless some means be taken to com- 
pensate for the drop in pressure caused by its resistance. 
Series boosters are frequently employed for this purpose. 
The generator element of the booster, in such a case, is 
connected, at the station or at any other point, in series 
with the feeder, the voltage-drop in which is to be compen- 
sated. As the drop is equal to the current, /, carried by 
the feeder times its resistance Rf, the voltage generated in 
the booster, E B , should be such that 

— = constant, 

a condition which is satisfied by a generator whose charac- 
teristic is a straight line, Fig. 1 82, making an angle a with 
the abscissae whose tangent is E B /I. Such a characteristic 




28o DYNAMO ELECTRIC MACHINERY. 

cannot be obtained, but, if the iron of the magnetic circuit 
be not fluxed above the knee of the magnetization curve by 
the maximum current to be carried by the feeder, a curved 
characteristic may be obtained which 
lies sufficiently close to the straight 
line to yield satisfactory results. 
The curve lies above the straight 
line and is concave towards it if the 
compensation is perfect at full load, 
o load curre~ i With boosters to be used on ordinary 
Flg ' l82- railroad circuits, the maximum vari- 

ation of voltage from that indicated by the straight line, is 
generally considered to be permissible, if it does not exceed 
10% of the maximum voltage supplied by the booster. 
As the field flux increases with the load, sparkless commu- 
tation is easily obtained at full load, and therefore copper 
brushes may advantageously be employed for the purpose 
of increasing the efficiency. 

The series booster is also frequently used in electric 
railway systems, which use the grounded rails for returning 
the propulsion current to the generating station. It is used 
to reduce the portion of the return current which would 
otherwise pass. through the earth and its substructures. It 
is then termed a negative or track-return booster. Consider 
a point on the track rail to be at a potential above the 
grounded terminal of the generator at the station and that 
the propulsion current is returned to this terminal, under 
this potential, by three paths connected in parallel, namely 
the track rails, the earth, and a negative feeder in series 
with a negative booster. Representing the currents flow- 
ing in the respective paths by I h I e , and I f , and the resist- 
ances by R h R e , and R-, then, if the voltage generated by 



BOOSTERS. 281 

the booster be Eb and if it be properly directed, the differ- 
ence of potential will be 



Rflt — Re^e ~ I 



<*-» 



The expression in parentheses represents the apparent resist- 
ance of the negative feeder. If the booster be so designed 
that the slope of its characteristic is approximately R f the 
apparent resistance of the negative feeder becomes zero, 
nearly all the current will return through it, and the earth 
currents will be reduced. 

Sometimes the field coil of the negative booster is con- 
nected in series with the outgoing feeders while the arma- 
ture is connected in series with the return feeder as above. 

Boosters are extensively used in connection with storage 
batteries for regulating the voltage between conductors of 
constant-potential systems. 

A so-called shunt booster, with its armature connected, in 
series with a battery, between the station bus-bars, and with 
its shunt field coils also connected through a regulating 
rheostat to the same bus-bars, is often used to increase the 
voltage impressed upon the battery above that between bus- 
bars during the charging of the battery. Since the charg- 
ing pressure per cell varies from 1.8 volts at the start to 
2.65 volts at heavy over-charge, if there be n cells and E volts 
between bus-bars, the booster will be required to furnish at 
a maximum an E.M.F. 

E B = 11(2.6$ — 1.8) = 0.85 n volts, 

and the current capacity must be the same as the maximum 
charging current of the battery. When such an arrange- 
ment is used the booster is employed only during the 
charging of the cells and not all the cells are used during 



282 DYNAMO ELECTRIC MACHINERY. 

the whole time of the discharge of the battery. A few 
cells, called end-cells, are cut out of circuit at the beginning 
of the discharge and are successively cut in again as the 
voltage per cell decreases due to the discharge. At the 
close of the discharge all the cells are in circuit and their 
number is 

E 

The E.M.F. capacity of the booster as well as its normal 
volt-ampere capacity accordingly amounts to about 40 % 
that of the battery. 

Frequently differentially-wound boosters, connected as 
shown in Fig. 183, are used in connection with generators 




Fig. 183. 

supplying energy both for lighting as well as for motors. 
This arrangement is especially suited to cases where the 
motor load fluctuates much while its average value is small 
and where a reduction of pressure upon the motor circuits 
under load is advantageous. Such cases are found in office 
buildings, apartment houses, and hotels, where a single gen- 
erator supplies energy for lamps as well as for elevator and 
pump motors. The motor bus-bars generally have 1 5 volts 
greater potential difference than the lighting bus-bars. 
At average motor load, the shunt ampere-turns predomi- 
nate, the series coil carries the average motor-load current^ 
the booster EM.F., E Bi is added to the generator E.M.F., 



BOOSTERS. 



283 



and the battery neither charges nor discharges. With 
heavy motor load, the series excitation falls slightly with a 
consequent fall of Eb, of motor bus-bar pressure, and of 
shunt excitation. The battery, therefore, supplies the ex- 
cess above the average of the motor-load current, while the 
main generator supplies, as before, this average. On light 
motor load Eb increases slightly, the motor bus-bar pres- 
sure rises, and the battery takes a charging current equal 
to the difference between the motor-load current and its 
average value. The current in the booster varies in prac- 
tice but a few per cent from the average motor-load cur- 
rent, and its direction is always the same. Such a machine 
is therefore called a non-reversible or a constant-current 
booster. 

In electric railway systems where there is a large aver- 
age current the battery charge and discharge rates are 
moderate and it is desirable that the voltage should not 
decrease with increase of load. In such cases the differ- 
entially-wound booster may be employed, with connections 
as indicated in Fig. 1 84, where the direction of the current 




Fig. 184. 



through the armature alters with change from charge to dis- 
charge of the battery. It is therefore called a reversible 
booster. At normal load, the series and the shunt ampere- 
turns are equal and opposed to each other, and hence the 



284 DYNAMO ELECTRIC MACHINERY. 

booster E.M.F., E B , is zero, the battery is neither charging 
nor discharging and its open-circuit voltage, E s , is equal to 
E, that of the generator and system. With heavy loads the 
series ampere-turns predominate, E B is added to E s , and the 
battery discharges. With light loads the shunt ampere- 
turns predominate, Eb opposes E s , and the battery charges. 
When the battery is discharged its E.M.F. falls, but the 
booster compensates therefor by taking more current in 
the series coil. The load on the generator is practically 
constant and the battery takes up the variations. The 
booster has to carry the maximum battery current and it 
must at the same time give its maximum E.M.F. ; and 
these values, therefore, determine its capacity. 

The series coils of the two differentially-wound boosters 
must be of sufficient cross-section to carry very large cur- 
rents, and this again requires such large magnet frames as 
to make the cost of the machines excessive. Means have 
therefore been devised for making use of shunt-wound 
boosters, the current in the shunt coil being changed by 
and in accordance with variations in the generator current. 



-AAAA/W 



Fig. 185. 

The Hubbard booster system makes use of an auxiliary 
generator, X, for furnishing exciting current for the booster, 
the connections being as indicated in Fig. 185. In prac- 
tice the exciter and driving motor are on one shaft. At 
average load the voltage of the exciter, E x , is the same as 



BOOSTERS. 



285 



that of the system, E, and opposed to it, the booster volt- 
age, E B , is zero, and the battery neither charges nor dis- 
charges. With heavy load E x > E, E B is added to E s , and 
the battery discharges. With light load, E X <E, E B is re- 
versed and is opposed to E s , and the battery charges. 
This system is sometimes called the counter E.M.F. system 
because the exciter E.M.F. opposes that of the system. 

Another much-used booster system employs the Entz 
carbon-plate regulator to control the exciting current in an 
auxiliary generator used as an exciter for the booster. The 




Fig. 186. 



connections are as indicated in Fig. 186. Two piles of 
carbon plates / and r have variable resistances, which are 
reduced under pressure, and whose magnitudes are con- 
trolled by the combined action of the solenoid 5 and the 
spring s. The resistance R { is reduced and R r increased 
upon increase of current in 5. At average load R t 
and R r are equal and therefore the point a is at the same 
potential as b, and there is no current in the field coil of 
the exciter X. Hence E x = E B = 0, E s = E, and the battery 
neither charges nor discharges. With heavy load R ( <:R r , 
E x >o, E B has the same direction as E s , and the battery 
discharges. With light load R r < R h E x > o, E B is opposed to 



286 DYNAMO ELECTRIC MACHINERY. 

E si and the battery charges. The appearance of the regu- 
lator is shown in Fig. 187. 

Booster armatures are generally lap-wound because of 
the large currents which they must carry. The current 
density in the windings can be made high because they are 




Fig. 187. 

rarely called upon to give their rated output. The reversible 
boosters should have laminated field-magnet cores in order 
to avoid a sluggish behavior. Sparking is liable to occur, 
unless the armature coils are of low reactance, because of 
the weak fluxing of the iron of the magnetic circuit. Safety 
relay devices should be used to prevent racing of the gen- 



STORAGE BATTERIES. 287 

erator element in case of the accidental opening of the shunt 
exciting circuit of the motor element. 

113. Storage Batteries. — Storage batteries are reversible 
electrolytic cells, whose electrodes are chemically modified 
by the passage of current through the cells and which 
thereby absorb and store energy when the current flows in 
one direction and give it up when the direction is reversed. 
The commercial forms make use of lead for electrodes and 
dilute sulphuric acid for an electrolyte. When charged, and 
energy has been absorbed, the positive electrode, that is the 
one of higher potential, is modified so as to contain an 
amount of lead peroxide (Pb0 2 ), while the other or neg- 
ative electrode contains a corresponding amount of sponge 
lead. 

Before a commercial cell can be considered as ready to 
receive its first or factory charge, the electrodes must have 
been materially modified from the condition of ordinary reg- 
uline lead. Their surfaces may have been rendered porous 
by mechanical and electrochemical treatment, or lead oxides 
may have been conductively united with them through me- 
chanical means. The plates constituting the former elec- 
trodes are termed Plante plates, while the latter are known 
as pasted plates. The purpose of the preliminary treatment 
ox formation of the electrodes is eventually to expose a large 
surface to the electrolyte, so that, under the limitations as 
to the velocities of the chemical reactions, a relatively high 
rate and large amount of energy may be absorbed during 
charge and be liberated during discharge. When fully 
charged, the porous portion of the positive electrode is 
peroxide of lead and of the negative is sponge lead. At all 
other times there is some lead sulphate present in the 
porous or active material of both electrodes. As lead sul- 



288 DYNAMO ELECTRIC MACHINERY. 

phate is a non-conductor of electricity, an excessive amount 
of it will interfere with the functioning of the active mate- 
rial. When the open-circuit potential of a cell sinks to 1.8 
volts the amount has increased to the permissible limit. 
The chemical changes taking place during charge and dis- 
charge are represented by the formula 

Charge 

Pb0 2 + Pb + 2 H 2 SO A = 2 PbS0 4 + 2 H 2 0. 



Discharge 

Plante plates are larger, heavier, more expensive, and 
more likely to be injured by impurities in the electrolyte 
than pasted plates, although they are more efficient, dura- 
ble, and dependable. They are best fitted for use in con- 
nection with central stations. Pasted plates are to be pre- 
ferred for motor-car propulsion. The electrochemical action 
does not penetrate much more than a millimeter below the 
surface of the electrode, because the active material has so 
much greater conductivity than the electrolyte and, at that 
depth, most of the current is confined to the active ma- 
terial, and there is, accordingly, no appreciable release of 
ions from the electrolyte. 

The acid of the electrolyte should be made from sulphur 
and not from pyrites and at full charge should be diluted 
to have a specific gravity of 1.20. During discharge the 
electrolyte gives up to the electrodes S0 3 and its specific 
gravity therefore falls from 1.13 to 1.19 depending upon 
the amount of electrolyte. Calculations of the resistance 
offered by the electrolyte can be based upon the assump- 
tion that its resistivity is 4/3 ohm per cubic centimeter at 
1 8° C. with a negative temperature coefficient of 0.016 per 
degree Centigrade. 



STORAGE BATTERIES. 



289 



The E.M.F. of a cell depends upon the concentration of 
the electrolyte and varies in accordance with the curve 
shown in Fig. 188. It also depends upon the condition of 



1. 1 
? 1 






















? n 






















1.9 
1 fl 






















/ 


/ 


















1.7 
1.6 

1 s 


/ 




















/ 








































1. 


00 


1.C 


5 


1. 

SPEC 




IFIC 


1. 

GRAV 


5 

ITY 


1.2 


1.25 



Fig. 188. 



charge. The terminal voltages during the hours of charge 
and discharge as a function of the time are shown in Fig. 1 89. 



2.6 
































































1 


2.4 
































1/ 












c 


HAR 


GE 












-^ 


S 




2.2 


































































2.00 












DIS 


;ha 


?GF 


















































1.8 




























X 


































\ 




1.6 






























\ 


, 




3 


I 




I 




HO 


4 
JRS 


t 






( 


) 






8 



Fig. 189. 



The E.M.F. of a fully charged cell is 2.5 volts. If an 
auxiliary cadmium electrode be inserted in the electrolyte 
its potential should be 2.3 volts below that of the positive 
plate and 0.2 volt above that of the negative plate. When 



29O DYNAMO ELECTRIC MACHINERY. 

fully discharged the E.M.F. of a cell is 1.8 volts and cad- 
mium should have a potential 2.05 below the positive elec- 
trode and 0.25 below the negative. By means of such a 
cadmium test the condition of either electrode can be deter- 
mined. It is common to take the average E.M.F. of a cell 
as 2 volts. To obtain a battery of greater E.M.F. a plural- 
ity of cells, connected in series, is employed. 

The capacity rating of a storage cell is expressed by the 
number of ampere-hours which it will furnish in discharging 
itself at constant current from a fully charged condition to 
a point where its potential on open circuit is 1.8 volts, the 
discharge being completed after the expiration of 8 hours. 
The actual ampere-hour capacity decreases with an increase 
of discharge rate, that is if made in less than 8 hours. It is 
reduced to one-half if made in one hour. The capacity is 
from 40 to 60 ampere-hours per square foot of exposed 
area of positive electrode, counting both sides but taking 
no account of increase of surface due to porosity. The 
normal current rate of charge or discharge is therefore from 
5 to 8 amperes per square foot. A continuous discharge 
should not exceed 25 amperes per square foot. Double this 
rate is permissible for 30 seconds or less. If it be desired to 
charge the cell rapidly the charging current should not be 
kept constant. To charge in three hours, for example, the 
current for the first hour should be 4 times the 8-hour rate, 
for the next 2.5 times, and for the last 1.5 times. Theoretic- 
ally the amount of lead chemically modified per ampere-hour 
on either electrode is 0.135 oz. Practically from 0.5 to 0.9 
oz. is required. There should be at least T V lb. of elec- 
trolyte for each ampere-hour. 

It is customary to connect numbers of positive plates 
by a lead lug to form the positive electrode of a cell and 



STORAGE BATTERIES. 



291 



to use one or more negative plates for the negative electrode. 
These are assembled in the electrolyte so that successive 
plates are of opposite polarity and both sides of each posi- 
tive plate are exposed to a negative. Containing jars are 




Fig. 190. 



made of glass or hard rubber, and for large cells lead-lined 
wooden tanks are used. Fig. 190 shows a 560-ampere-hour 
cell in a glass jar. The power output per pound of complete 
cell is from 8 to 14 watts with pasted plates and from 3 to 7 
watts with Plante plates. 



292 DYNAMO ELECTRIC MACHINERY. 

PROBLEMS. 

i . A dynamotor, the resistances of whose armature windings 
are each o. i ohm, is used as an equalizer on a 3-wire equivolt- 
age system with 200 volts between outside conductors. One 
ampere flows through the armatures when the system is bal- 
anced. Find the power expended in frictions and the counter 
E.M.F. of each armature winding. 

2. Find the regulation of each side of the system of the pre- 
ceding problem if the maximum unbalanced load be 100 am- 
peres. 

3 . A motor-generator, with one armature having a resistance 
-two-thirds as great as the other and designed to generate at 
the same speed two-thirds the E.M.F. generated by the other, 
is used as a balancer between outer wires having a potential dif- 
ference of 500 volts. The power required to overcome frictions 
is 400 watts and the resistance of the two armatures in series is 
1.2 ohms. A current of 150 amperes flows through the middle 
wire. Determine the capacity of each unit of the balancer. 

4. A 3-element four-wire balancer is used on a multivoltage 
variable-speed motor system with 40, 120, and 80 volts between 
successive wires. The armature resistances are proportional to 
the E.M.F. generated in them, and together amount to 0.48 
ohm. On balanced load the armatures take 10 amperes. If the 
current between successive line wires be 30, 75, and 10 amperes 
respectively, what are the respective currents in the armatures 
of the motor-generator elements ? What is the pressure between 
successive wires ? 

5. From a point one mile distant there returns to the generat- 
ing station for a single-track railway, that uses 70-lb. (per yard) 
rails, 200 amperes by way of the rails. Two parallel No. 0000 
copper wires in series with a negative booster and returning 
300 amperes to the station, will reduce to zero the drop between 
this point and the station with what booster voltage ? The re- 
sistivity of the rails, including bonds, is no ohms per mil-foot. 



PROBLEMS. 



293 



6. In Fig. 191 the curve represents a maximum daily 
load-curve, on a 500-volt system, variations in which are to be 
provided for by a storage battery. Determine the current / av 
such that the charging ampere-hours shall exceed the quantity 
of discharge by 10%, and then obtain the ampere-hour capacity 
and the exposed area of positive plates per cell and the number 
of cells in an appropriate battery having end-cell regulation. 




10 12 2 

HOURS 
Fig. 191. 



7. At 500 volts with battery arranged as in Fig. 184, how 
many cells would be required in the battery ? 

8. Plot a power output-time curve of the reversible booster of 
the preceding problem, the load curve being that shown in Fig. 
191; each cell having an ampere-hour capacity as in problem 
6, the voltage-hour curves of charge and discharge being those 
shown in Fig. 189, and the cell being considered as fully charged 
at the instant when the load first assumes its average value. 

9. If the booster and its driving motor each have an overload 
capacity of 25% for two hours and have an efficiency of 90% 
at full load, what is the capacity of each in kilowatts ? What is 
the maximum motor input ? 



294 DYNAMO ELECTRIC MACHINERY. 



CHAPTER IX. 
CENTRAL-STATION EQUIPMENT. 

114. Paralleling of Generators. — In general a generator 
is much more efficient when operated at its full load than 
when operated at one-half or one-quarter load. It is usual 
to install in central stations, which, as a rule, must supply 
different quantities of electrical energy at different times of 
the day, a number of smaller units rather than one unit 
large enough to supply the total energy. By this means 
any load can be handled by a machine or by a number of 
machines all operating at about their maximum efficiency. 
It is necessary, therefore, to consider the methods of com- 
bining two or more machines to supply energy to a single 
load. 

The simplest and most usual method of connecting 
generators is that employed in incandescent light generat- 
ing stations, where a number of constant-pressure machines 
are connected in parallel, the positive and negative terminals 
of each generator being connected respectively to positive 
and negative common conductors, called bits-bars, which are 
located on the rear of the operating switchboard. The 
connections of two shunt-wound machines with their regu- 
lating rheostats are shown in Fig. 192. The various load 
external circuits are connected in parallel to the bus-bars. 
This practice is frequently modified by separating those 
machines which supply energy to the more distant loads 
from those that supply the shorter circuits. This is because 



CENTRAL-STATION EQUIPMENT. 



=95 



the maintaining of a constant and uniform pressure at all 
distributing points requires a higher pressure on the station 
ends of the longer mains than that on the shorter. When 
a machine is to be connected to bus-bars to which other 
operating machines are already connected, it is first brought 



Fig. 192. 



up to speed ; the field magnetization is then adjusted till 
the machine generates the same voltage as that of the bus- 
bars, and the main switch is then closed, which puts the 
machine in circuit. The proper voltage at which to connect 
in the new machine may be roughly determined by compar- 
ing the relative brightness of its pilot lamp with that of the 
lamps operating on the circuit. A more exact way is to 
compare the readings of a voltmeter across the terminals of 
a machine with one across the bus-bars. Another method 
is to connect the generator to the bus-bars through a high 
resistance and a galvanometer indicator. When the latter 
indicates no deflection, the voltages of the machine and the 
bus-bars are identical, and the machine may be connected 
in. Sometimes the differential indicator is used for this 
purpose. 

When shunt machines are connected in parallel, their volt- 
ages should be maintained the same so that the total load 



296 DYNAMO ELECTRIC MACHINERY. 

may be properly apportioned among them. If this equality 

of voltage be not maintained, no serious damage will occur, 

jince the machine which generates the lower voltage merely 

fails to take its full share of the load. Even if the voltage 

of one machine falls so low that it is overpowered and run 

as a motor, still no damage will result, save perhaps the 

blowing of a fuse, since the direction of rotation for a shunt 

machine is the same whether it be run as a generator or as 

a motor. If it be desired to regulate a number of machines 

simultaneously by one regulator, it may be accomplished by 

bringing the positive ends of the field coils to one terminal 

of the regulator and connecting the other terminal to the 

negative bus. 

Shunt machines may be operated in series by connecting 

the positive brush of one machine to the negative brush of 

j-^-i + the next, and connecting the 

g /^x j extreme outside brushes with 

s - / \ " >i 

the line wires. Each machine 
can be regulated separately to 
generate any portion of the 
pressure, or, if it be desired to 
regulate all the machines thus 
Fi s- *93- connected uniformly and as a 

unit, the field coils of all the machines may be joined 
in series with one regulating rheostat, and shunted across 
the line wires. Fig. 193 illustrates such an arrangement 
for two 115-volt generators. 

Series-wound generators may be operated in series, as in 
the Thury system of direct-current high-potential power 
transmission, which is in use on a number of lines in 
Europe. The aggregate capacity of the 1 5 plants at pres- 
ent employing this system is 25,000 horse-power, the line 



CENTRAL-STATION EQUIPMENT. 297 

from Montiers to Lyons being the longest (112 miles) and 
employing the highest voltage (57,000 volts). 

There are a number of groups of generators at the power 
house connected in series, each group being driven by a 
water turbine or other prime mover ; the voltage per 
machine being less than 4000. Each generator is insulated 
from ground and from the other machines of the group, 
the middle point of the system being grounded to limit the 
required insulation. The line current is maintained con- 
stant by several complicated auxiliary devices. These auto- 
matically regulate the speed of all the prime movers, so as 
to keep the line voltage proportional to the load; cut in 
or out of circuit one or more machines if there be large 
changes in load ; and short-circuit any disabled machine. In 
the substations, a number of series-wound motors are con- 
nected in series across the line, the motors being arranged 
in groups, each group driving a generator. The generators, 
which may deliver either direct or alternating current, are 
connected in multiple for distribution. The motors and 
generators in the substations are insulated from each other 
and from ground, just as are the machines in the generating 
station. The current taken by the motors is kept constant 
by an automatic shifting of the brushes. The Thury sys- 
tem is adapted only to undertakings where the power is to 
be transmitted over a long distance and the load is to be 
concentrated at few points, since at every tap a complete 
substation must be provided containing motors having an 
aggregate voltage equal to the line voltage. If the line 
alone be considered, direct-current power transmission is far 
superior to alternating-current transmission, which is em- 
ployed exclusively in this country. Less conductor mate- 
rial is required and none of the disturbing influences met 



298 



DYNAMO ELECTRIC MACHINERY. 



with in alternating-current transmission are encountered in 
the Thury system. 

Difficulty is experienced if it be attempted to operate 
series generators in parallel. If the machines start with a 
proper distribution of load among them, and if one generates 
slightly less than its full voltage, then this machine does 
not continue to take its full share of the load; and, since it 
is series wound, the magnetic field becomes weakened, thus 
resulting in a still lower voltage. The conditions continually 
grow more uneven until the machine is overpowered and 
it becomes a motor. Since the direction of rotation of a 
series-wound motor is opposite to its direction when run as 
a generator, serious results may occur. One way in which 
this difficulty may be overcome is to arrange the field coils 
so that the magnetization in any one machine will remain 
the same as in the other machines, even though its pressure 
falls below that of the others. To accomplish this result 
the series fields must all be placed in parallel. This may 
be done by means of an equalizer, which is a wire of small 
resistance connecting all of the brushes of one polarity, and 




Fig. 194- 



placing the field in parallel, as shown in Fig. 194. Two 
series dynamos may be run in parallel without an equalizer 
by resorting to mutual excitation, that is, by letting the cur- 



CENTRAL-STATION EQUIPMENT. 



299 



1 



& 



SHUNT 
JM1 



ERIES 

(POP 






rent of one armature excite the field of the other. In this 
case, if the pressure of one machine falls and its load there- 
fore decreases, the magnetization of the other is reduced, 
compelling the first to maintain its share of the load. 
Series dynamos are never operated in parallel in prac- 
tice, but this discussion is introduced because of its 
application to the parallel operation of compound-wound 
dynamos. 

Compound-wound generators are extensively used for 
constant-potential distribution. Since these machines have 
series field coils as well as shunt field coils, the parallel 
connection thereof for combined output should involve an 
equalizing bus, as in Fig. 195. Any number of compound 
generators may be op- 
erated in parallel regard- 
less of the size of the 
units, provided their volt- 
ages are the same and 
that the resistances of 
their series field coils are 
inversely proportional to 
the currents supplied by the individual machines. Over- 
compounded generators which are to be operated in parallel 
must yield exactly the same voltage increase at full load 
or at any other load. This may be adjusted by interposing 
additional resistance in the series field coil of that generator 
which carries more than its share of the load. 

115. Parallel Operation of Motors. — Any number of 
shunt motors may be placed in parallel across constant- 
pressure mains, and their operation will be satisfactory 
whether each has a separate load or whether they be con- 
nected through suitable devices to a common shaft. Shunt 



FIELD COILS 



Fig. 195- 



300 DYNAMO ELECTRIC MACHINERY. 

motors will operate in series on a constant-pressure circuit 
when positively coupled together; but if connected to the 
same shaft by belts, and one belt slips or comes off, that 
motor will race, and receive more than its proper portion 
of the voltage. This arrangement is not common. 

Series motors will operate satisfactorily on constant- 
pressure circuits if rigidly coupled to their loads. Series 
motors connected in series on constant-pressure mains will 
operate satisfactorily, dividing up the total voltage between 
them according to the load each is carrying. If it be 
desired to make them share a load equally they must be 
geared together so that each rotates at the speed correspond- 
ing to its share of the voltage. Series motors only are used 
on constant-current circuits. Any number of these may be 
placed in series on such a circuit individually or connected 
to a common shaft. A series motor on a constant-current 
circuit may be overloaded until it stops without harm, since 
a constant current flows at any speed. 

Compound-wound motors are coming into quite general 
use, and they are invariably operated on constant-pressure 
circuits, and each machine has its own load. 

116. Switches. — Switches are devices inserted in a cir- 
cuit to facilitate its establishment or interruption. They 
are generally of the form known as knife switches, and may 
be single-pole, double-pole, etc., according to the number 
of circuit interruptions simultaneously effected. Fig. 196 
shows a 3000-ampere 125-volt double-throw back-connected 
multiple-blade knife switch made by the Anderson Manu- 
facturing Company. 

The metal parts of such switches consist of copper hinges, 
blades, and clips, and these must be properly designed to 
have sufficient current-carrying capacity and contact surface. 



CENTRAL-STATION EQUIPMENT. 



301 



It is usual to allow one square inch of cross-sectional area 
for every 800 to 1000 amperes, and to provide one square 
inch of contact surface for every 60 to 75 amperes of cur- 
rent. The distances between metal parts of opposite 
polarity and the break distances of approved switches, in 




Fig. 196. 

inches, are given in the following table. Frequently 
switches are provided with fuse connections, and also lugs 
into which the ends of the leads are soldered. 





MINIMUM SEPARATION 










OF METAL PARTS OF 


MINIMUM BREAK DISTANCE 




OPPOSITE POLARITY 








SIZE OF 
SWITCH 




















125 V. 


125 v. 


250 V. 


125 V. 


125 v. 


250 V. 




OR 


TO 


TO 


OR 


TO 


TO 




LESS 


250 V. 


600 V. 


LESS 


250 V. 


600 V. 


ic amp. or less 


I 


1% 


3% 


K 


!#■ 


3 


10-35 amperes 


I# 


tii 


4 


I 


I& 


3% 


35-100 " 


i}4 


2^ 


AVz 


iK 


2 


4 


100-300 " 


*H 


2 ^ 




2 


2^ 




300-600 " 


2% 


2% 




2% 


2}<; 




600-1000 " 


3 


3 




2% 


-% 





302 



DYNAMO ELECTRIC MACHINERY. 




Fig. i97- 



A ioooo ampere 500 volt " rotary" switch for use on 
central-station switchboards is shown in Fig. 197. 

117. Fuses. — Fuses are devices intended to protect cir- 
cuits from destruction or damage which might result from an 
excessive flow of current through them. They are made 



CENTRAL-STATION EQUIPMENT. 303 

of fusible material, generally of lead or an alloy of tin and 
lead, and take the form of wire or strips provided at each 
end with a copper terminal which is slotted to fit into fuse 
receptacles. 

The magnitude of the current which will melt a fuse de- 
pends upon the length of the wire. Short lengths of a wire 
of given cross-section and given material will carry larger 
currents than longer lengths. The heat which is generated 
in the short ones escapes more rapidly, owing to the prox- 
imity of large masses of metal which commonly form the 
terminals of the fuse. Fuses are rated at 80 per cent of 
the greatest current they can carry indefinitely without 
melting. This rating enables the fuse to carry a current 
25 per cent greater than the normal current for which the 
fuse is designed. 

The "blowing" of a fuse is accompanied by a flash and 
a spattering of the fused metal ; this may ignite near-by 
combustible materials. Therefore link fuses should be 
placed in separate porcelain or other fireproof receptacles. 
The better practice is to use enclosed fuses, in which the 
fusible conductor is surrounded by a finely divided powder 
contained in an insulating casing. 

118. Circuit Breakers. — -In central-station practice the 
fuse with its uncertainties has been superseded by the 
electro-magnetic circuit breaker. This device acts promptly 
and definitely, and has the advantage that a circuit once 
opened by it, due to an excessive current therein, may be 
instantly reestablished. 

A circuit breaker consists of a switch which may be 
closed against the action of a strong spring and kept closed 
by means of a latch. This latch is controlled by the 
plunger of a solenoid which is connected in series with the 



304 



DYNAMO ELECTRIC MACHINERY. 



line. When an abnormal current flows through the circuit, 
the plunger is attracted and strikes against a trigger which 
releases the latch. The spring then becomes operative and 
the circuit breaker is opened. The opening of a circuit 
carrying an excessive current is accompanied by an arc 
across the break. To avoid arcing across the metal con- 
tacts of a circuit breaker, this device is provided with an 
auxiliary set of carbon contacts so arranged that the latter 
are opened an instant later than the main metal contacts. 
Thus all the sparking takes place at the renewable carbon 
contacts. 

A Westinghouse single-pole 500-volt circuit breaker for 
use with compound-wound generators is shown in Fig. 198. 

The solenoid is a massive 
copper coil located on the 
rear of the marble panel. 
The movable member is built 
up of thin sheets of spring 
copper which contact edge- 
wise against solid copper ter- 
minal blocks. This instru- 
ment may be adjusted to 
open the circuit which it pro- 
tects for any predetermined 
current strength between the 
limits of 20 % less than, and 
50 % in excess of, the normal 
current. 

When generators are operated in parallel, each machine 
should be protected by a reverse-current circuit breaker, 
which will open when a reverse current of predetermined 
value flows. The usual overload circuit breaker, just de- 




Fig. 198. 



CENTRAL-STATION EQUIPMENT. 305 

scribed, may be used for this purpose when provided with 
a relay ; or, better still, a circuit breaker equipped with two 
solenoids may be used, one of which operates on the passage 
of an excessive current and the other on the flowing of a 
reverse current. A circuit breaker especially suitable for 
storage-battery work to prevent the current from flowing 
back through the generator is the underload circuit breaker. 
It is usually adjusted to open the circuit when the current 
falls below one-tenth of its normal rated value. Another 
type of circuit breaker is that having a no-voltage release ; 
this instrument is particularly adapted for the protection 
of motor circuits from dangers accruing from the circuit 
remaining closed while the line is idle. 

119. Measuring Instruments. — The instruments em- 
ployed in direct-current work are ammeters, voltmeters and 
wattmeters, for measuring respectively current, voltage and 
power. Every instrument has some movable part to which 
a pointer, passing over a divided scale, is attached. Two 
forces act upon this moving element, one causing a deflec- 
tion thereof from its zero position, and the other, opposing 
the first, limits the deflection so that the position of the 
moving element when in equilibrium gives a proper indica- 
tion of the magnitude of the deflecting force. 

Ammeters. There are four types of commercial amme- 
ters: (1) those in which the force-action between two coils 
carrying current serves as a measure of that current, (2) 
those in which the force-action between a permanent mag- 
net and a coil carrying current is utilized as a measure of 
the current, (3) those in which the amount of attraction of 
a soft -iron core or vane by a coil carrying current serves 
as an indication of the current strength, and (4) those 
in which the expansion of a wire heated by the passage 



306 DYNAMO ELECTRIC MACHINERY. 

of a current through it is utilized as a measure of the 
current. 

The electrodynamometer is an ammeter of the first type, 
and consists of two coils connected in series, one coil being 
fixed and the other movable. The planes of these coils are 
normally at right angles to each other, but when a current 
flows through them they tend to place themselves in the 
same plane. This tendency of rotation of the movable coil 
is resisted by a torsional spring. The angle through which 
the spring is turned in order to restore the coil while carry- 
ing the current to its original position is measured by means 
of a pointer and a dial. If a be the angle turned through, 
then the current strength is 

/ = k Va, 

where k is a constant determined by calibration. 

Ammeters of the second type may have the coil fixed 
and the permanent magnet movable, but the moving-coil 
instruments, such as the Weston meter, are more generally 
used. The Weston instrument consists of a coil composed 
of a large number of turns of fine insulated copper wire 
wound on a light rectangular frame of copper or aluminum, 
this coil being pivoted in jeweled bearings and mounted in 
an annular space between the poles of a permanent magnet 
and a soft-iron core at the center, as shown in Fig. 199. 
Two spiral springs serve to carry the current to and from 
the moving coil, and also control the amount of deflection. 
The current strength is directly proportional to the angle 
of deflection, and therefore the scales of such instruments 
are uniform over the entire range. 

Ammeters in which a soft-iron piece is attracted by 
an electromagnet carrying the current to be indicated are 



CENTRAL-STATION EQUIPMENT. 



307 




Fig. 199. 



most generally used for the measurement of alternating 
currents, but serve equally well for direct currents. In the 
most approved form 
of this type of instru- 
ment the current to 
be measured passes 
through a fixed coil 
and thereby magne- 
tizes a soft -iron vane 
which is pivoted and 
controlled by a spring. 
Magnetizing the vane 
causes it to move, 
the amount of move- 
ment indicating the 
current strength on the properly calibrated scale. 

The principle of operation of the hot-wire type of amme- 
ter is that the heat produced by a current which traverses 
a wire having a negligible temperature coefficient of resist- 
ance is proportional to the square of that current, and, since 

the temperature rise of the wire 
_ is proportional to the heating, 
and the linear expansion of the 
wire is proportional to the tem- 
6 perature rise, it follows that the 

^-^ \ linear expansion is directly pro- 

portional to the square of the 
current value. The expansion 
of the wire may be observed by a pointer, P, which passes 
over a calibrated scale, the arrangement being as shown 
in Fig. 200. 

Frequently only a small part of the whole current to be 




3 o8 



DYNAMO ELECTRIC MACHINERY. 



measured passes through the ammeter coil, the remainder 
flowing through a by-path of low resistance, called a shunt. 
The resistance of a shunt for a given instrument is so pro- 
portioned that a full-scale deflection will be produced when 
a specified current flows through the shunt. The size of 
the shunt is designated by this current value. If it be 
proposed to use a moving-coil instrument, which gives a 
full-scale deflection on E volts, to indicate a maximum cur- 
rent of / amperes, then the resistance of the shunt must 

be — ohm. 

Voltmeters. Most 



very high resistance 



voltmeters are simply ammeters of 
They may therefore be connected 
to supply mains without 
causing more than a slight 
flow of current through the 
instrument. The resist- 
ance of voltmeters is in the 
neighborhood of ioo ohms 
per volt of maximum scale 
deflection. The appearance 
of switchboard voltmeters 
and ammeters is shown in 
Fig. 201. 

Fi s- 201 - Another type of volt- 

meter, depending for its operation on the attraction between 
two electrically charged bodies, is the electrostatic volt- 
meter. Low-voltage instruments of this type consist of a 
number of thin plates horizontally suspended between 
corresponding quadrants, and fitted with a pointer which 
plays over a divided scale. 

In order to vary the range of the usual type of voltmeters, 




CENTRAL-STATION EQUIPMENT. 



309 



resistance is connected in series with the instruments, such 
resistances being known as multipliers. If R be the resist- 
ance of a low-reading voltmeter, and r be that of the multi- 
plier, then the range of the instrument has been increased 
r+R 



R 



times. Multipliers should be wound non-inductively 



with wire having a negligible temperature coefficient of 
resistance. 

Wattmeters. The power delivered to direct-current re- 
ceiving circuits may be determined from voltmeter and 
ammeter readings, or may be measured directly by means 
of a wattmeter. This instrument consists of a fixed coil, 
which is connected in series with the load circuit, and a 
movable coil, which is connected in series with a high re- 
sistance across the supply mains, as shown in Fig. 202. 




Fig. 202. 



The deflecting force is proportional to the product of the 
currents flowing in the two windings, but the current flow- 
ing in the movable coil is proportional to the voltage of the 
circuit; therefore the deflecting force is proportional to 
the product of the current supplied and the voltage, or to 
the power delivered to the load. Wattmeters are used 
principally in connection with alternating-current measure- 
ments. 

To measure the energy delivered to a circuit, watt-hour 



3io 



DYNAMO ELECTRIC MACHINERY. 



meters are employed. In order that an instrument may 
record the time element, some part of the meter must move 
constantly through unit distance for each unit of energy 
delivered, and this movement must be permanently recorded 
by a suitable device such as a dial train. The Thomson 
watt-hour meter, shown in Fig. 203, consists of a spherical 

armature rotating within 
two circular field coils, one 
on either side of the arm- 
ature. This instrument is 
connected to the circuit in 
the same manner as the 
indicating watt-meter. The 
armature is carried by a 
vertical spindle, the lower 
end of which rests in a jew- 
eled bearing, and the upper 
end is provided with a worm 
which meshes with a chain 
of wheels constituting the 
counting mechanism. The 
torque produced is propor- 
tional to the product of the 
field flux and the armature current. As there is no iron in 
the magnetic circuit and since the speed of rotation is low, 
little or no counter E.M.F. will be induced in the armature 
coil; thus the armature current is independent of the speed, 
and is directly proportional to the line voltage. For the 
same reason the field flux is proportional to the main current. 
Therefore the torque is proportional to the power consumed 
by the load. In order that the meter shall record correctly 
it is only necessary to provide some means for making the 




CENTRAL-STATION EQUIPMENT. 



311 



speed of rotation proportional to the torque. This is accom- 
plished by applying a magnetic drag, in the form of an 
aluminum disk fastened to the armature spindle and passing 
between the poles of permanent magnets. The electromo- 
tive forces induced in the disk are proportional to the 
number of lines of force cut in a given time, and, since the 
resistance of the disk is constant, the strength of the eddy 
currents will be proportional to the rate of cutting lines of 
force, and consequently will vary with the speed of rotation. 
The drag or counter torque, being proportional to the prod- 
uct of the constant flux and the eddy currents, will vary 
directly with the speed. To overcome mechanical friction 
an auxiliary field coil or starting coil is provided which 
consists of a few turns 
of fine wire connected 
in series with the arma- 
ture. 

Instruments for re- 
cording successive in- 
stantaneous values of 
current, voltage or 
power are called record- 
ing instruments. They 
operate on the same 
principles as indicating 
instruments, and are 
made recording by fit- 
ting the pointer with an 
ink pen which presses against a paper chart wound 
on a drum, the latter rotating slowly at constant speed 
by clockwork. A curve-drawing wattmeter is shown in 
Fig. 204. 




Fig. 204. 



312 DYNAMO ELECTRIC MACHINERY. 

120. Switchboards. — The object of a central-station 
switchboard is to group the necessary devices for controlling, 
distributing and measuring the current received or delivered, 
particular attention being directed toward locating these 
devices for convenient operation. Safety apparatus for pro- 
tecting generators or the lines against abnormal conditions 
are sometimes placed upon the switchboard. 

Switchboards are designed with a view to a symmetrical 
arrangement of the apparatus and instruments, and it is 
usual to place all similar devices in the same horizontal row. 
. As a rule circuit breakers are located at the top of the 
board and recording instruments at the bottom. Measuring 
instruments are placed at such height as to be conveniently 
read by the switchboard attendant. Rheostats may be 
placed above or below the switchboard at the rear, but the 
handles controlling them, through the agency of chains and 
sprocket wheels, must be located so that the attendant can 
manipulate them and note the instrument indications simul- 
taneously. The bus-bars and the connections from them to 
the switches, circuit breakers, rheostats and to the instru- 
ments are mounted on the rear of the switchboard. 

Switchboards are constructed so that each panel controls 
the apparatus for a single generator or controls a definite 
number of feeders. Fig. 205 illustrates a switchboard for 
two compound-wound generators, and consisting of two 
generator panels and one feeder panel for four feeders. 
Each generator may be connected to the bus-bars by means 
of a main switch and a circuit breaker. The current sup- 
plied to the load by each generator is measured by a 
separate ammeter and shunt, but the voltage of both ma- 
chines is measured alternately by a single voltmeter which 
may be connected to either machine by a voltmeter switch 



CENTRAL-STATION EQUIPMENT. 



313 



<h> 




CIRCUIT 
BREAKER 



[^ 



3=3 



KH^K°>i 




<D- 



CIRCUIT 
BREAKER 




?=+3 



55 



IS 



OH 



a 



cM 



j 



(H 



9 



9 

! 3 L 



9 
! 4 







"-O 



£ 



i 



— / 6EN. 




W 



+ m 




Fig. 205. 



IMMM^ 



314 DYNAMO ELECTRIC MACHINERY. 

on the feeder panel. The rheostats on the generator panels 
regulate the excitation of the generator shunt fields. In 
order that these compound-wound machines may be oper- 
ated in parallel, equalizer switches and an equalizer bus-bar 
are necessary. A lamp on each panel provides illumination 
for the scales of the measuring instruments; the lamps on 
the generator panels also serve as pilot lamps. The outer 
lamps on the feeder panel constitute a ground detector, and 
are used for indicating grounds. Should a partial ground 
occur on one line the corresponding lamp would burn dimly 
and the other brightly, thus indicating by their relative 
brightness the extent of the fault. 

121. Works Cost. — The costs of the various items 
which are involved in the manufacture of dynamo-electric 
machines are so dependent upon the types of the specific 
designs that a comprehensive discussion of them cannot be 
given in this book. Commercial designers, however, are 
responsible for the manufacture of machines which are as 
inexpensive as is consistent with satisfactory operation. 
Although, with a machine of given speed and output, the 
detail costs for labor, iron, copper and insulating material 
may vary widely in different cases, the total works costs 
are not very different. If the diameter and over-all length 
of the armature be D and / inches respectively, then the 
works cost in dollars may be expressed as 

Cost = KDl, 

where K is a function of the speed and output of the 
machine. This function is fairly constant for pressures 
between 100 and 500 volts, for the extra costs of insulation 
in the construction of the machines of higher voltage are 
compensated for by the reduced costs of commutator copper. 



CENTRAL-STATION EQUIPMENT. 



315 



The values of K may be obtained from the curves shown 
in Fig. 206, which correspond to peripheral velocities, v, of 
the armature of 200, 150, and 100 or less feet per second. 
The increased values for velocities above 100 are due to 
increased costs of labor and of suitable material for with- 
standing the higher centrifugal forces. 

































V ==L. 


A50^> 


















VjZ- 


100 of 


_tESS__ 















































400 601 

OUTPUT IN K.W. 

Fig. 206. 



1000 



122. Selling Prices. — The selling price of a machine 
must exceed the works cost by an amount sufficient to 
include the profit and such expenses as litigation, advertis- 
ing, and sales commissions or expenses. It is customary for 
manufacturing concerns to print in their commercial publi- 
cations list-prices of machines which considerably exceed 
the total costs of them when delivered. Substantial dis- 
counts are allowed to purchasers, amounting to as much 
as 50% in some cases, and depending upon the extent of 
purchase, date of payment and many other complex condi- 
tions. The actual selling price per K.W. is dependent upon 
the speed, output and type of machine. To give a general 
idea as to the selling prices of compound-wound generators 
the curves of Figs. 207 and 208 are given. The prices are 
not far from those which would be paid by unfavored pur- 



3i6 



DYNAMO ELECTRIC MACHINERY. 





































125-VOLT 
COMPOUND GENERATORS 




\ 


























YS 






























V^ 


5 ^ 














\c 


$ 

?* 










^eo 












$ 




•^ 




s^T 


















^ 


— S 
























O CAPA 


DITY 





























































500 z 



100 150 

CAPACITY IN K.W. 



Fig. 207. 



5 

o 20 

















250-VOLT 
ENG1NE-TYFE 
















COMPOUND GENERATORS 
AT 100 R.P.M. 



















































































CAPACITY IN K.W. 



Fig. 208. 



CENTRAL-STATION EQUIPMENT. 



317 



chasers in 19 10. In Fig. 209 are given the selling prices 
of 230-volt shunt or series motors for normal speeds of 
1000 revolutions per minute, and of 125-volt balancers. 



















































^^53-1/0, . 
















s^» 






^U^ 


CER S 














^tr 


^OTOf 


lATt 


]00 R.p_ 


M. 

































































CAPACITY IN K.W. 

Fig. 209. 



a. 






















O^ 

lie 












































































0. "* 























10C0 2CC0 30C0 4000 

AMPERE-HOUR CAPACITY AT 8-HOUR RATE 

Fig. 210. 



The selling prices of lead-lead-sulphuric acid storage cells 
per ampere-hour of capacity at an 8-hour rate are em- 
bodied in the curve of Fig. 210 as functions of the 
capacities. 



3 18 DYNAMO ELECTRIC MACHINERY. 

123. Plant Costs. — A fair average cost per kilowatt for 
a steam-driven power plant is $100, although with steam- 
turbine plants it may be placed at $80. For hydraulic 
plants the cost is greater than these, and has been estimated 
at $200 per kilowatt. These prices are applicable to central 
stations of reasonably large capacity. In such stations 
alternating-current generators are usually employed. Di- 
rect-current generators are more generally used in isolated 
plants, such as in office buildings or in manufactories, where 
steam is generated for heating or power purposes and its 
use for driving electrical generators is subsidiary and inci- 
dental. In such cases the cost of delivery and of erection 
of generators may be estimated as 60 cents per K.W. 
The various installation costs may be estimated from the 
data given by Stott in the following table: 

POWER PLANT COST PER K.W. 

Minimum Maximum 

i . Real Estate $3 . 00 $7 . 00 

2. Excavation .75 1.25 

3. Foundations, Reciprocating Engines 2.00 3.00 

4. Foundations, Turbines .50 .75 

5. Iron and Steel Structure 8.00 10.00 

6. Building (Roof and Main Floor) 8.00 10.00 

7. Floors, Galleries and Platforms 1 . 50 2.50 

8. Tunnels, Intake and Discharge 1 .40 2 .80 

9. Ash Storage Pocket, etc .70 1 . 50 

10. Coal-hoisting Tower 1 . 20 2 . 00 

11. Cranes .40 .60 

1 2. Coal and Ash Conveyors 2 . 00 2.75 

13. Ash Cars, Locomotives and Tracks .15 .30 

T4. Coal and Ash Chutes, etc .40 1 . 00 

15. Water Meters, Storage Tanks and Mains . .50 1 .00 

16. Stacks 1.25 2 . 00 

1 7 . Boilers 9.50 n.50 

18. Boiler Setting 1.25 1.75 



CENTRAL-STATION EQUIPMENT. 319 

Minimum Maximum 

19. Stokers $1 . 30 $2.20 

20. Ec jnomizers 1.30 2.25 

21. Flues, Dampers and Regulators .60 .90 

22. Forced Draught Blowers, Air Ducts, etc . . . 1.25 1.65 

23. Boiler Feed and Other Pumps .40 .75 

24. Feed-water Heaters, etc .20 .35 

25. Steam and Water Piping, Traps, Separators, 

High and Low Pressure 3 . 00 5 .00 

26. Pipe Covering .60 1 . 00 

27. Valves .60 1 . 00 

28. Main Engines, Reciprocating 22 .00 30.00 

29. Exciter Engines, Reciprocating .40 .70 

30. Condensers, Barometric or Jet 1 .00 2 .50 

31. Condensers, Surface 6.00 7.50 

32. Electric Generators 16.00 22.00 

33. Exciters .60 .80 

34. Steam Turbine Units Complete 22 .00 32 .00 

35. Rotaries, Transformers, Blowers, etc .60 1 .00 

36. Switchboards Complete 3. 00 3. 90 

37. Wiring for Lights, Motors, etc .20 .30 

38. Oiling System Complete .15 .35 

39. Compressed Air System and Other Small 

Auxiliaries .20 .30 

40. Painting, Labor, etc 1.25 1.75 

41. Extras 2 .00 2 .00 

42. Engineering Expenses and Inspection 4.00 6.00 

Costs of Excavation near New York City 

Earth $2.44 per cu. yd. 

Rock 6 . 00 per cu. yd. 

Brickwork 1 1 . 30 per cu. yd. 

124. Operating Expenses There are many items which 

make up the expenses for operating a plant and for making 
repairs which are essential for maintaining the machinery 
in good running condition. The relative costs of these 
items have been given by Stott as listed in the following 
table : 



320 



DYNAMO ELECTRIC MACHINERY. 



DISTRIBUTION OF MAINTENANCE AND OPERATING 
CHARGES. 










„ H 












z a z 




f? z 






in 
*Z 


P < 5 

u W P 


h 


Z 03 

§5* 




2 5 


W ffl 


O H h 


a 1 


Z Z H 




x z 


H « 


« Z „ 




W < 






E D 


* £ S 


t/> h 


w s 




u 

a 


h 


Sgs 





< < 
w 




X 




« w h 




H 
(/3 


Maintenance 












i. Engine Room Mechanical . . 


2.57 


0.5I 


i-54 


2-57 


I.54 


2. Boiler Room or Producer Room 


4.61 


4.30 


3-5 2 


115 


i-95 


3. Coal- and Ash-handling Appa- 












ratus 


O.58 


o-54 


0.44 


O.29 


0.29 


4. Electrical Apparatus .... 


1. 12 


1. 12 


1. 12 


1. 12 


i.I2 


Operation 












5. Coal- and Ash-handling Labor . 


2.26 


2. 11 


1.74 


i-i3 


113 


6. Removal of Ashes .... 


I.06 


0.94 


0.80 


o.53 


O.5O 


7. Dock Rental 


O.74 


0.74 


0.74 


0.74 


O.74 


8. Boiler-room Labor .... 


7.15 


6.68 


546 


1.79 


303 


9. Boiler-room Oil, Waste, etc. 


O.I7 


0.17 


0.17 


0.17 


O.T7 


10. Coal . ... 


61.30 


57-3Q 
0.71 


46.87 
546 


26.31 
3.57 


2577 
2.14 


11. Water 


7.14 


12. Engine-room Mechanical Labor 


6.71 


i-35 


4.03 


6.71 


4.03 


13. Lubrication 


1.77 


o.35 


1. 01 


1.77 


I.06 


[4. Waste, etc 


O.30 


0.30 


0.30 


0.30 


O.30 


15. Electrical Labor 


2.52 


2.52 


2.52 


2.52 


2.52 


Relative Cost of Maintenance and 












Operation 


100. 00 


79.64 


75-72 


50.67 


46.32 


Relative Investment in per cent 


IOO.OO 


82.50 


77.00 


100.00 


9I.20 



125. Cost of Electrical Energy The cost attendant 

upon the delivery of electrical energy at the bus-bars of a 
central station is made up of two factors. 

The first is constant in magnitude, is independent of the 
amount of delivered energy, and appears in the station 
records as a fixed charge. It includes such items as interest 
on the investment, insurance, taxes, depreciation, and obso- 
lescence. The last item is one which is due to the frequent 



CENTRAL-STATION EQUIPMENT. 



321 



improvements in central station apparatus, which make it 
desirable at times to cast aside operative machines before 
they are worn out, in order to take advantage of the greater 
efficiency afforded by newer types. Dr. C. T. Hutchinson 
gives the following approximate values for the items of fixed 
charges expressed as percentages of the cost of the plant : 

FIXED CHARGES OF GENERATING PLANTS. 





STEAM 


WATER 


Interest 

Insurance 

Taxes 

Depreciation 

Obsolescence 


6.0% 
5-o 


6.0% 

1.0 
J-5 


i7-o% 


9.0% 



The second factor is the maintenance and operating 
expense, which depends upon the amount of electrical energy 
delivered to the bus-bars and varies with it. Although this 
expense per kilowatt-hour varies somewhat with the ratio 
of the load to the maximum capacity of the plant, it may 
safely be estimated at 0.5 cent. 

The average cost throughout a day or year of a unit of 
delivered electrical energy, therefore, depends upon the ratio 
of the average power to the maximum power. This ratio 
is termed the load factor of the plant. Inasmuch as all 
generators have an overload capacity, due to their ability 
to store heat for a limited time without an excessive result- 
ant rise of temperature, the maximum power is really taken 
as the average power for say an hour during the period of 
maximum load. It is common for load factors to be as low 
as 10%, although that of the Interborough Rapid Transit 
Company varies between 50% and 55 %. The influence 
of the load factor upon the cost of a kilowatt-hour of 



322 



DYNAMO ELECTRIC MACHINERY. 



electrical energy is shown in the following table, the cal- 
culations being based upon a iooo-K.W. steam-turbine 
plant costing $80 per kilowatt and operating continuously 
throughout the year, viz. for 8760 hours. 

EFFECT OF LOAD FACTOR ON COST OF POWER. 



COSTS 


LOAD FACTORS 


0.2 


0.4 


0.6 


0.8 


1.0 


Fixed Charges . . 


0.77 R 


0.388 

0.500 


0.258 

0.500 


0.194 

0.500 


°-i55 

0.500 


Operating Expenses . . 
Total per Kilowatt-hour in 


Cents . 


0.50O 


1-275 


0.888 


0.758 


0.694 


o-655 



PROBLEMS. 

1. Design a good 500-ampere, no-volt, double-pole, single- 
throw, back-connected switch, but use no more material than 
necessary. How many pounds of copper are required if the 
terminal studs be 5 inches long ? 

2. A moving-coil instrument gives a full-scale deflection on 
0.068 volt. What is the resistance of a 200-ampere shunt 
therefor ? 

3. The resistance of a 150-volt voltmeter is 14,000 ohms. 
What must be the resistance of a multiplier for this instrument 
so that it can measure E.M.F.'s up to 750 volts? 

4. A watt-hour meter without a starting coil reads correctly 
when run on a i-K.W. load. On light load a power consump- 
tion of 100 watts will just start the armature rotating. What 
will be the meter indication after running i\ hours on a constant 
load of 600 watts, assuming running friction as one-half of 
starting friction ? 

5. Lay out the connections of a suitable switchboard for an 
isolated plant having a single compound-wound generator, which 
is intended to supply three feeder circuits. 



INDEX. 



Absolute electrical units, 3. 

Action of a generator, principle of, 45. 

motor, principle of, 210. 
Acyclic dynamos, 184. 
Ageing of iron, 41. 
Air gap, magnetic distribution in, 

112, 119. 
Allis-Chalmers Co. generators, 177. 
Alternator, the, 46. 
Ammeters, 305. 
Ammeter shunts, 308. 
Ampere, definition of, 3. 

-hour, definition of, 3. 

-turn, definition of, 32. 

-turns, exciting, calculation of, 96. 
for compensating armature re- 
action, 116. 
Anderson Mfg. Co. switch, 300. 
Angle of brush lead or lag, 113, 216. 
Applications of shunt motors, '236. 
Arc-light generators, 85. 
Armature bearings, 85. 

coils, 73. 

copper loss, 148. 

core construction, 67. 

cores, losses in, 146. 

equalizing connections, 65. 

of dynamo, 52. 

reaction, no, 216. 

compensation for, 115. 

shafts, 83. 

slots, 72. 

windings, 55. 
Automobile motors, 258. 
Auxiliary field poles, 1 20. 

Balancers, 274. 

Ball-bearings for armatures, 86. 
Batteries, storage, 287. 
Bearing friction, 151. 
Bearings, armature, 85. 
Bipolar field magnets, 54. 



Boosters, 279. 

Braking, dynamic, 262. 

Brushes, 81. 

Brush arc-light generator, 193. 

holders, 82. 

lead or lag, 113, 216. 

pressure, 80. 

transition resistance, 79. 
Burke Electric Co. three- wire gen- 
erator, 184. 

Calculation of exciting ampere- 
turns, 96. 

of reactance voltage, 127. 
Capacity of a dynamo, 140. 
Cast iron, magnetic properties of, 34. 
Characteristic curves of generators, 
162, 172, 187. 

of motors, 233, 241, 247. 
Circuit breakers, 303. 
Circular mil, definition of, 6. 
Coefficient, economic, 158. 

of conversion, 158. 

of dispersion, 95. 

of mutual induction, 25. 

output, 145. 

self-induction, 24. 
Coercivity, 39. 
Commercial efficiency, 159. 
Commutating plane, 113. 

poles, 120, 219. 
Commutation, conditions for spark- 
less, 134. 

frequency of, 126. 

process of, 121. 

self-inductance, 122, 129. 

time of, 125. 
Commutator construction, 76. 

function of, 46. 

losses, 79, 152. 
Compensation for armature re- 
action, 115. 



3 2 3 



324 



INDEX. 



Compound excitation, 92. 
Compounding, 171. 
Compound-wound generators, see 
Generators. 
motors, 263. 
Conductivity, definition of, 6. 
Conductors, resistance of, 5. 
Constant-current booster, 283. 
distribution, 160. 
-potential distribution, 160. 
Control of motor speed, 218. 

of railway motors, 251. 
Copper loss in armature coils, 148. 
Core construction, 67. 
Cost of dynamos, 314. 
of electrical energy, 320. 
of operating machine tools, 238. 
of plants, 318. 
- of storage batteries, 317. 
Coulomb, definition of, 3. 
Counter E.M.F. of motors, 214. 
Crane motors, 261. 
Crocker-Wheeler mill motor, 260. 

motor for lathes, 237. 
Cross-magnetizing effect of arma- 
ture current, in. 
Current, absolute unit of, 3. 
density in field coils, 107. 
Cutler-Hammer Mfg. Co. con- 
troller, 263. 
rheostat, 169. 
starting and field rheostat, 228. 

Decay of current in inductive cir- 
cuit, 27. 

Degree of re-entrancy, 64. 

Demagnetizing effect of armature 
current, 113. 

Density of flux, 14. 

Design of starting rheostats, 230. 

Dettmar's three- wire generator, 182. 

Devices for reducing armature re- 
action, 118. 

Diamagnetic substances, 21. 

Dielectric strength, n. 
test of, 13. 

Difference of potential, 3. 

Differential boosters, 282. 
motor, 263. 

Direction of induced E.M.F. , 23. 
of rotation of motors, 211. 

Dispersion coefficient, 95. 



Distribution, constant-current, 160. 
-potential, 160. 

three- wire, 181. 
Divided circuits, 8. 
Dobrowolsky's three-wire gener- 
ator, 182. 
Ducts for ventilation, 70. 
Dynamic braking, 262. 
Dynamo capacity, 140. 

homopolar, 184. 
Dynamos, 45. 

E.M.F. equation of, 66. 

heating of, 142. 
Dynamotors, 266. 
Dyne, definition of, 1. 

Economic coefficient, 158. 
Eddy currents, 42. 
losses due to, 147. 
Electric Mfg. Co. automobile 
motor, 259. 
Effect of load factor on cost of 

power, 322. 
Efficiency of dynamos, 155. 

of motors, 234. 
Electrical distribution, 160. 
efficiency, 159. 
energy, cost of, 320. 
units, 3. 
Electro-Dynamic Co. dynamo field 
structure, 121. 
motor, 221. 
Electrodynamometers, 306. 
Electro-magnetic induction, 21. 
Electromotive force fluctuation, 
52. 
generated, 48. 
induced in conductor, 23. 
of dynamo, 66. 
Element of winding, 56. 
Elevator, motor-driven, 238. 
End-cell control, 282. 
Energy, definition of, 1. 
Entz regulator, 285. 
Equalizers, 269, 298. 
Equalizing connections, 65. 
Erg, definition of, 1. 
Excelsior arc-light generator, 198. 
Excitation loss, 151. 

of field magnets, 92. 
Exciting ampere-turns, calculation 
of, 96. 



INDEX 



325 



Expenses, operating, 319. 
External characteristic, 187. 

Feeders, 166. 
Field coils, 53, 104. 

current density in, 107. 

cores, 88. 

excitation, 92. 

magnet frames, 88, 180. 

magnets, 53. 

rheostats, 166. 
Flat compounding, 172. 
Fleming's Rule, 23, 211. 
Fluctuation of E.M.F., 52. 
Flux density, 20. 
Foot-pound, definition of, 1. 
Force, definition of, 1. 
Foucault currents, 42. 
Frequency, 52. 

of commutation, 127. 
Function of commutator, 46. 
Fuses, 302. 

Gauss, definition of, 21. 
General Electric Co. constant-cur- 
rent generator, 193. 
constant-potential generator, 

174, 179, 184. 
field rheostat. 167. 
railway motor, 247. 
shop-tool controller, 229. 
Generated electromotive force, 48. 
Generator losses, 146. 
output of, 145. 
principle of action of, 45. 
Generators, compound-wound, char- 
acteristic curves of, 172. 
efficiency of, 155. 
field excitation of, 92. 
for lighting and railways, 173. 
homopolar, 184. 
parallel operation of, 294. 
rating of, 145. 
series- wound, 189. 

characteristic curves of, 187. 
shunt-wound, characteristic curves 
of, 162. 
regulation of, 164. 
split-pole, 118. 
three- wire, 180. 
Gilbert, definition of, 32. 
Ground detectors, 314. 



Growth of current in inductive cir- 
cuit, 26. 

Hamilton motor-operated drill press, 

236. 
Hand regulation, 165. 
Heat developed by a current, 11. 
Heating of dynamos, 142. 
Henry, definition of, 25. 
Homopolar generators, 184. 
Horse-power, definition of, 2. 
Hot-wire instruments, 307. 
Hubbard booster system, 284. 
Hysteresis loss in armature cores, 

147. 
magnetic, 38. 
Hysteretic constants, 42. 

Induction, 20. 

electro-magnetic, 21. 

mutual, 25. 

self, 24. 
Inductors, armature, 56. 
Industrial applications of shunt 

motors, 236. 
Instruments, 305. 
Insulating materials, n. 
Insulation resistance, n. 
Intensity of magnetic field, 19. 
Interpoles, 120, 219. 
Iron loss in armature cores, 147. 

Joule, definition of, 1. 

Kinetic energy, definition of, 2. 
Kirchhoff's Laws, 9. 

Laminations of armature core, 68. 

Lap armature windings, 57. 

Leakage, magnetic, 94. 

Lighting generators, 173. 

Lines of force, 18. 

Load factor of plants, 321. 

Loading-back method of testing 

motors, 235. 
Losses in armature coils, 148. 

cores, 146. 
in commutator and brushes, 153. 
in field pole faces, 149. 

winding, 151. 
Lubrication, 85. 
Lundell generator, 118, 179. 



326 



INDEX. 



Machine-tool operation, cost of, 

238. 
Magnetic distribution in air gap, 

112, 119. 
Magnetic field, definition of, 18. 
intensity of, 19. 
hysteresis, 38. 
leakage, 94. 
permeability, 20. 
potential, 20. 

properties of iron and steel, 34. 
Magnetization curves, ^$- 
Magneto, 92, 159. 
Magnetomotive force, 32. 
Magnet pole, strength of, 18. 
Matthiessen standard resistivity, 7. 
Maxwell, definition of, 21. 
Measuring instruments, 305. 
Mechanical units, 1. 
Meters, recording watt-hour, 310. 
Mill motors, 260. 

Motor, armature reactions of, 216. 
counter E.M.F. of, 214. 
direction of rotation of, 211. 
-generators, 273. 
power of, 216. 
principle of action of, 210. 
torque exerted by, 213. 
Motors, compound-wound, 263. 
field excitation of, 92. 
parallel operation of, 299. 
series, 239. 

characteristic curves of, 241. 
for automobiles, 258. 
for rolling mills, 260. 
on railways, 243. 
shunt, characteristic curves of, 233. 
industrial applications of, 236. 
interpole, 221. 
speed, 217. 

control of, 222, 277. 
regulation of, 232. 
starting of, 225. 
Multiple-circuit armature windings, 

57- 
-unit railway motor control, 255. 

Multiplex armature windings, 62. 

Multipliers, 309. 

Multipolar field magnets, 55. 

Multivoltage distribution system, 

222. 

Mutual induction, 25. 



Negative booster, 280. 

Neutral of three- wire system, 181. 

plane, 115. 
Non-reversible booster, 283. 

Oersted, definition of, 37. 
Ohm, definition of, 4. 
Ohm's Law, 4. 
Operating expenses, 319. 
Otis traction elevator, 238. 
Output coefficients, 145. 
Over-compounding, 172. 

Parallel connection of circuits, 8. 

operation of motors, 299. 
Paralleling of generators, 294. 
Paramagnetic substances, 21. 
Permeability, 20. 
Permeance, 37. 
Pitch, pole, 90, 

winding, 59. 
Plane of commutation, 113. 
Polar span, 91. 
Pole-face losses, 149. 

pitch, 90. 

shoes, 88. 
Potential energy, definition of, 2. 

magnetic, 20. 
Poundal, definition of, 1. 
Pound as a unit of force, 1. 
Power, definition of, 2. 

lines, 189. 

of electric current, 10. 

of motors, 216. 

plant costs, 318. 

transmission, Thury system, 296. 
Practical electrical units, 3. 
Pressure, definition of, 4. 
Principle of generator action, 45. 

of motor action, 210. 
Problems, 16, 43, 86, 108, 138, 207, 

265, 292, 322. 
Protective devices in circuits, 302. 



Quantity of electricity, 3, 29. 

Railway generators, 173. 
motors, 243. 

characteristic curves of, 247. 
control of, 251. 



INDEX. 



327 



Rating of machines, 145. 
Reactance voltage, 123. 
calculation of, 127. 
Reaction of armature currents, no. 
compensation for, 115. 
devices for reducing, 118. 
Recording instruments, 311. 
Re-entrant armature windings, 63. 
Regulation of series generators, 190. 

speed of shunt motors, 232. 

voltage of generators, 164. 
Reluctance, 36. 
Reluctivity, definition of, 37. 
Resistance, 4. 

specific, 6. 

temperature coefficient of, 7. 
Resistivity, definition of, 5. 
Retentivity, 39. 
Reversible booster, 283. 
Rheostatic railway controller, 251. 
Rheostats, field, 166. 

starting, 225. 
design of, 230. 
Rolling mills, motors for, 260. 

Saturation, 33. 
Self-induction, 25. 

-regulation, 172. 
Selling prices of machines, 315. 
Series-booster, 279. 

connection of circuits, 7. 

excitation, 92. 

-parallel railway controller, 252. 

-wound generators, see Generators. 
Shafts of armatures, 83. 
Shop-tool controller, 229. 
Short-chord armature windings, 62. 
Shunt-booster, 281. 

excitation, 92. 

motors, see Motors. 
Shunts for ammeters, 308. 
Shunt-wound generators, see Gen- 
erators. 
Simplex armature windings, 56. 
Slots in armature cores, 72. 
Space factor of armature slots, 74. 

of field coils, 107. 
Sparkless commutation, conditions 

for, 134. 
Specific resistance, 6. 
Speed control of shunt motors, 218. 

regulation of shunt motors, 232. 



Split-pole type of generator, 118. 
Starting rheostats, 225. 

design of, 230. 
Steam-turbine driven generator, 

179. 
Steel, magnetic properties of, 35. 
Storage batteries, 287. 

cost of, 317. 
Stow Mfg. Co. motor, 221. 
Strength, dielectric, n. 

of magnet pole, 18. 
Suspension of railway motors on 

trucks, 250. 
Switchboards, 312. 
Switches, 300. 

Table of armature shaft fits, 174. 
cost of machine-tool operation, 

238. 
current paths in armature wind- 
ings, 67. 
dielectric strengths, 14. 
fixed charges of plants, 321. 
flux densities in dynamos, 97. 
generator speeds, 141, 174. 
hysteretic constants, 41 . 
load factor versus cost of power, 

322. _ 
magnetic properties of iron and 

steel, 35. 
number of dynamo field poles, 

142. 
operating and maintenance 

charges, 320. 
power plant cost, 318. 
resistivities, 6. 
switch data, 301. 
test voltages, 16. 
Temperature coefficient of resist- 
ance, 7. 
elevation of dynamos, 143, 153. 
Thomson-Houston arc-light gen- 
erator, 200. 
watt-hour meter, 310. 
Three-wire generators, 180. 
Thury system of power transmis- 
sion, 296. 
Time of commutation, 125. 
Toroids, 32. 

Torque exerted by motors, 213. 
Track-return booster, 280. 



328 



INDEX. 



Unipolar dynamos, 184. 

Variable-speed control of motors, 218. 
Ventilating ducts in core, 70. 
Voltage regulation of shunt gen- 
erators, 164. 
Volt, definition of, 3. 
Voltmeter multipliers, 309. 
Voltmeters, 308. 

Ward Leonard motor control system, 
223. 
self-starter, 229. 

Watt, definition of, 2. 

Wattmeters, 309. 

Wave armature windings, 57. 

Western Electric Co. arc-light gen- 
erator-, 204. 



Western Electric Co. generator, 

176. 
Westinghouse circuit breaker, 304. 

field rheostat, 168, 169. 

generator, 178. 

railway controller, 254. 
motor, 243. 
Weston instruments, 306. 
Windage, 151. 
Winding pitch, 59. 
Windings, armature, 55. 
Work, definition of, 1. 
Works cost, 314. 

Wrought iron, magnetic properties 
of, 34- 

Yoke, 54. 



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SHELDON, S., and HAUSMANN, E. Dynamo-Electric Machinery: Its 
Construction, Design, and Operation. 
Vol. I.: Direct-Current Machines. Eighth Edition, completely re-written. 
Illustrated. 8vo., cloth, 310 pp Net, $2.50 

SHELDON, S., MASON, H., and HAUSMANN, E. Alternating-Current 
Machines: Being the second volume of "Dynamo-Electric 
Machinery; its Construction, Design, and Operation." With many 
Diagrams and Figures. (Binding uniform with Volume I.) 
Seventh Edition, rewritten. 8vo., cloth, 353 pp Net, $2.50 

SLOANE, T. O'CONOR. Standard Electrical Dictionary. 300 Illustra- 
tions. 12mo., cloth, 682 pp $3.00 

Elementary Electrical Calculations. A Manual of Simple Engineer- 
ing Mathematics, covering the whole field of Direct Current 
Calculations, the basis of Alternating Current Mathematics, Net- 
works, and typical cases of Circuits, with Appendices on special 
subjects. 8vo., cloth. Illustrated. 304 pp Net, $2.00 

SNELL, ALBION T. Electric Motive Power. The Transmission and Dis- 
tribution of Electric Power by Continuous and Alternating Currents. 
With a Section on the Applications of Electricity to Mining Work. 
Second Edition. Illustrated. 8vo., cloth, 411 pp Net, $4.00 



SODDY, F. Radio-Activity ; an Elementary Treatise from the Stand- 
point of the Disintegration Theory. Fully Illustrated. 8vo., cloth, 
214 pp Net, $3.00 

SOLOMON, MAURICE. Electric Lamps. Illustrated. 8vo., cloth. (Van 
Nostrand's Westminster Series.) Net, $2 .00 

STEWART, A. Modern Polyphase Machinery. Illustrated. 12mo., 
cloth, 296 pp Net, $2 .00 

SWINBURNE, JAS., and WORDINGHAM, C. H. The Measurement of 
Electric Currents. Electrical Measuring Instruments. Meters for 
Electrical Energy. Edited, with Preface, by T. Commerford Martin. 
Folding Plate and Numerous Illustrations. 16mo, cloth, 241 pp. 
(No. 109 Van Nostrand's Science Series.) 50 cents 

SWOOPE, C. WALTON. Lessons in Practical Electricity: Principles, 
Experiments, and Arithmetical Problems. An Elementary Text- 
book. With numerous Tables, Formulae, and two large Instruction 
Plates. Eleventh Edition, revised. Illustrated. 8vo., cloth, 462 pp. 

Net, $2.00 

THOM, C, and JONES, W. H. Telegraphic Connections, embracing recent 
methods in Quadruplex Telegraphy. 20 Colored Plates. 8vo., 
cloth. 59 pp $1 .50 

THOMPSON, S. P. Dynamo-Electric Machinery. With an Introduction 
and Notes by Frank L. Pope and H. R. Butler. Fully Illustrated. 
16mo., cloth, 214 pp. (No. 66 Van Nostrand's Science Series.) 

50 cents 

Recent Progress in Dynamo-Electric Machines. Being a Supplement to 
"Dynamo-Electric Machinery." Illustrated. 16mo., cloth, 113 pp. 
(No. 75 Van Nostrand's Science Series.) 50 cents 

TOWNSEND, FITZHUGH. Alternating Current Engineering. Illus- 
trated. 8vo., paper, 32 pp Net, 75 cents 

UNDERHILL, C. R. Solenoids, Electromagnets and Electromagnetic 
Windings. 218 Illustrations. 12mo., cloth, 345 pp Net, $2.00 

URQUHART, J. W. Dynamo Construction. A Practical Handbook for 
the use of Engineer Constructors and Electricians in Charge. Illus- 
trated. 12mo., cloth $3.00 



Electric Ship-Lighting. A Handbook on the Practical Fitting and Run- 
ning of Ship's Electrical Plant, for the use of Ship Owners and Build- 
ers, Marine Electricians, and Sea-going Engineers in Charge. 88 
Illustrations. 12mo., cloth, 308 pp $3.00 

Electric-Light Fitting. A Handbook for Working Electrical Engineers, 
embodying Practical Notes on Installation Management. Second 
Edition, with additional chapters. With numerous Illustrations. 
12mo., cloth $2.00 

Electroplating. A Practical Handbook. Fifth Edition. Illustrated. 
12mo., cloth, 230 pp $2.00 

Electrotyping. Illustrated. 12mo., cloth, 228 pp $2.00 

WADE, E. J. Secondary Batteries: Their Theory, Construction, and Use. 
Second Edition, corrected. 265 Illustrations. 8vo., cloth, 501 pp. 

Net, $4.00 

WADSWORTH, C. Electric Battery Ignition. 15 Illustrations. 16mo. 
paper In Press 

WALKER, FREDERICK. Practical Dynamo-Building for Amateurs. 
How to Wind for any Output. Third Edition. Illustrated. 16mo., 
cloth, 104 pp. (No. 98 Van Nostrand's Science Series.) 50 cents 

WALKER, SYDNEY F. Electricity in Homes and Workshops. A 
Practical Treatise on Auxiliary Electrical Apparatus. Fourth Edition. 
Illustrated. 12mo., cloth, 358 pp $2 .00 

Electricity in Mining. Illustrated. 8vo., cloth, 385 pp $3.50 

WALLING, B. T., and MARTIN, JULIUS. Electrical Installations of the 
United States Navy. With many Diagrams and Engravings. 8vo., 
cloth, 648 pp $6 .00 

WALMSLEY, R. M. Electricity in the Service of Man. A Popular and 
Practical Treatise on the Application of Electricity in Modern Life. 
Illustrated. 8vo., cloth, 1208 pp Net, $4.50 

WATT, ALEXANDER. Electroplating and Refining of Metals. New 
Edition, rewritten by Arnold Philip. Illustrated. 8vo., cloth, 677 
pp Net, $4.50 

Electrometallurgy. Fifteenth Edition. Illustrated. 12mo., cloth, 225 
pp $1.00 



WEBB, H. L. A Practical Guide to the Testing of Insulated Wires and 
Cables. Fifth Edition. Illustrated. 12mo., cloth, 118 pp $1 .00 

WEEKS, R. W. The Design of Alternate-Current Transformer. 

New Edition in Press 

WEYMOUTH, F. MARTEN. Drum Armatures and Commutators. 
(Theory and Practice.) A complete treatise on the theory and con- 
struction of drum-winding, and of commutators for closed-coil arma- 
tures, together with a full resume of some of the principal points 
involved in their design, and an exposition of armature reactions 
and sparking. Illustrated. 8vo., cloth, 295 pp Net, $3 .00 

WILKINSON, H. D. Submarine Cable Laying, Repairing and Testing. 
Second Edition, completely revised. 313 Illustrations. 8vo., cloth, 
580 pp Net, $6.00 

YOUNG, J. ELTON. Electrical Testing for Telegraph Engineers. Illus- 
trated. 8vo., cloth, 264 pp Net, $4.00 

ZEIDLER, J., and LUSTGARTEN, J. Electric Arc Lamps: Their Princi- 
ples, Construction and Working. 160 Illustrations. 8vo., cloth, 
188 pp Net, $2.00 




A 96=page Catalog of Books on Electricity, classified by 
subjects, will be furnished gratis, postage prepaid, 
on application. 



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